fledgeForGpt Module Docs (#4089)

* fledgeForGpt Module Docs

Add documentation for the experimental FLEDGE support module.

* Update fledgeForGpt.md

adding layout header

* relative links, related reading

Co-authored-by: jdwieland8282 <wieland.jeff@gmail.com>
Co-authored-by: bretg <bgorsline@gmail.com>

dev-docs/modules/fledgeForGpt.md

@@ -0,0 +1,154 @@
+---
+layout: page_v2
+page_type: module
+title: Module - fledgeForGpt
+description: how to use fledge with GPT
+module_code : fledgeForGpt
+display_name : Fledge for GPT
+enable_download : true
+sidebarType : 1
+---
+
+# Overview
+This module allows Prebid.js to support FLEDGE by integrating it with GPT's [experimental FLEDGE
+support](https://github.com/google/ads-privacy/tree/master/proposals/fledge-multiple-seller-testing).
+
+To learn more about FLEDGE in general, go [here](https://github.com/WICG/turtledove/blob/main/FLEDGE.md).
+
+This document covers the steps necessary for publishers to enable FLEDGE on their inventory. It also describes
+the changes Bid Adapters need to implement in order to support FLEDGE.
+
+## Publisher Integration
+Publishers wishing to enable FLEDGE support must do two things. First, they must compile Prebid.js with support for this module.
+This is accomplished by adding the `fledgeForGpt` module to the list of modules they are already using:
+
+```
+gulp build --modules=fledgeForGpt,...
+```
+
+Second, they must enable FLEDGE in their Prebid.js configuration. To provide a high degree of flexiblity for testing, FLEDGE
+settings exist at the module level, the bidder level, and the adunit level.
+
+### Module Configuration
+This module exposes the following settings:
+
+|Name |Type |Description |Notes |
+| :------------ | :------------ | :------------ |:------------ |
+|enabled | Boolean |Enable/disable the module |Defaults to `false` |
+
+As noted above, FLEDGE support is disabled by default. To enable it, set the `enabled` value to `true` for this module
+using the `setConfig` method of Prebid.js:
+
+```js
+pbjs.que.push(function() {
+  pbjs.setConfig({
+    fledgeForGpt: {
+      enabled: true
+    }
+  });
+});
+```
+
+### Bidder Configuration
+This module adds the following setting for bidders:
+
+|Name |Type |Description |Notes |
+| :------------ | :------------ | :------------ |:------------ |
+| fledgeEnabled | Boolean | Enable/disable a bidder to participate in FLEDGE | Defaults to `false` |
+
+In addition to enabling FLEDGE at the module level, individual bidders must also be enabled. This allows publishers to
+selectively test with one or more bidders as they desire. To enable one or more bidders, use the `setBidderConfig` method
+of Prebid.js:
+
+```js
+pbjs.setBidderConfig({
+    bidders: ["openx"],
+    config: {
+        fledgeEnabled: true
+    }
+});
+```
+
+### AdUnit Configuration
+Enabling an adunit for FLEDGE eligibility is accomplished by setting an attribute of the `ortb2Imp` object for that
+adunit.
+
+|Name |Type |Description |Notes |
+| :------------ | :------------ | :------------ |:------------ |
+| ortb2Imp.ext.ae | Integer | Auction Environment: 1 indicates FLEDGE eligible, 0 indicates it is not | Absence indicates this is not FLEDGE eligible |
+
+The `ae` field stands for Auction Environment and was chosen to be consistent with the field that GAM passes to bidders
+in their Open Bidding and Exchange Bidding APIs. More details on that can be found
+[here](https://github.com/google/ads-privacy/tree/master/proposals/fledge-rtb#bid-request-changes-indicating-interest-group-auction-support)
+In practice, this looks as follows:
+
+```js
+pbjs.addAdUnits({
+    code: "my-adunit-div",
+    // other config here
+    ortb2Imp: {
+        ext: {
+            ae: 1
+        }
+    }
+});
+```
+
+## Bid Adapter Integration
+Chrome has enabled a two-tier auction in FLEDGE. This allows multiple sellers (frequently SSPs) to act on behalf of the publisher with
+a single entity serving as the final decision maker. In their [current approach](https://github.com/google/ads-privacy/tree/master/proposals/fledge-multiple-seller-testing),
+GPT has opted to run the final auction layer while allowing other SSPs/sellers to participate as
+[Component Auctions](https://github.com/WICG/turtledove/blob/main/FLEDGE.md#21-initiating-an-on-device-auction) which feed their
+bids to the final layer. To learn more about Component Auctions, go [here](https://github.com/WICG/turtledove/blob/main/FLEDGE.md#24-scoring-bids-in-component-auctions).
+
+The FLEDGE auction, including Component Auctions, are configured via an `AuctionConfig` object that defines the parameters of the auction for a given
+seller. This module enables FLEDGE support by allowing bid adaptors to return `AuctionConfig` objects in addition to bids. If a bid adaptor returns an
+`AuctionConfig` object, Prebid.js will register it with the appropriate GPT ad slot so the bidder can participate as a Component Auction in the overall
+FLEDGE auction for that slot. More details on the GPT API can be found [here](https://developers.google.com/publisher-tag/reference#googletag.config.componentauctionconfig).
+
+Modifying a bid adapter to support FLEDGE is a straightforward process and consists of the following steps:
+1. Detecting when a bid request is FLEDGE eligible
+2. Responding with AuctionConfig
+
+FLEDGE eligibility is made available to bid adapters through the `bidderRequest.fledgeEnabled` field.
+The [`bidderRequest`](/dev-docs/bidder-adaptor.html#bidderrequest-parameters) object is passed to
+the [`buildRequests`](/dev-docs/bidder-adaptor.html#building-the-request) method of an adapter. Bid adapters
+who wish to participate should read this flag and pass it to their server. FLEDGE eligibility depends on a number of parameters:
+
+1. Chrome enablement
+2. Publisher participatipon in the [Origin Trial](https://developer.chrome.com/docs/privacy-sandbox/unified-origin-trial/#configure)
+3. Publisher Prebid.js configuration (detailed above)
+
+When a bid request is FLEDGE enabled, a bid adapter can return a tuple consisting of bids and AuctionConfig objects rather than just a list of bids:
+
+```js
+function interpretResponse(resp, req) {
+    // Load the bids from the response - this is adapter specific
+    const bids = parseBids(resp);
+
+    // Load the auctionConfigs from the response - also adapter specific
+    const auctionConfigs = parseAuctionConfigs(resp);
+
+    if (auctionConfigs) {
+        // Return a tuple of bids and auctionConfigs. It is possible that bids could be null.
+        return {bids, auctionConfigs};
+    } else {
+        return bids;
+    }
+}
+```
+
+An AuctionConfig must be associated with an adunit and auction, and this is accomplished using the value in the `bidId` field from the objects in the
+`validBidRequests` array passed to the `buildRequests` function - see [here](/dev-docs/bidder-adaptor.html#ad-unit-params-in-the-validbidrequests-array)
+for more details. This means that the AuctionConfig objects returned from `interpretResponse` must contain a `bidId` field whose value corresponds to
+the request it should be associated with. This may raise the question: why isn't the AuctionConfig object returned as part of the bid? The
+answer is that it's possible to participate in the FLEDGE auction without returning a contextual bid.
+
+An example of this can be seen in the OpenX OpenRTB bid adapter [here](https://github.com/prebid/Prebid.js/blob/master/modules/openxOrtbBidAdapter.js#L327).
+
+Other than the addition of the `bidId` field, the `AuctionConfig` object should adhere to the requirements set forth in FLEDGE. The details of creating an
+`AuctionConfig` object are beyond the scope of this document.
+
+## Related Reading
+- [FLEDGE](https://github.com/WICG/turtledove/blob/main/FLEDGE.md)
+- [Component Auctions](https://github.com/WICG/turtledove/blob/main/FLEDGE.md#21-initiating-an-on-device-auction)