Add order book documentation (#85)

* add order book doc

* Apply suggestions from code review

Co-authored-by: Malte Kliemann <mail@maltekliemann.com>

* use unified order book wording

* mention market creator fees

* remove unnecessary sentence

* add price example description

* Update docs/learn/order-book.md

Co-authored-by: Malte Kliemann <mail@maltekliemann.com>

* change phrasing

---------

Co-authored-by: Malte Kliemann <mail@maltekliemann.com>

docs/learn/order-book.md

@@ -0,0 +1,86 @@
+---
+id: order-book
+title: Order Book
+---
+
+An order book is a digital ledger used in asset trading that records open
+orders, a buy or sell instruction from a trader for a specific asset or
+security. It functions as the backbone of most modern trading platforms and
+exchanges, enabling users to execute trades based on current market conditions.
+
+## How the Order Book works in Zeitgeist
+
+In Zeitgeist, the order book contains trading pairs of outcome assets and the
+market's base asset (collateral). The order book can be set as the market's
+scoring rule. The operations within the order book are primarily managed through
+three fundamental functions: `place_order`, `remove_order`, and `fill_order`.
+It's important to mention that market creator fees are charged for each taken
+order.
+
+### Place Order
+
+This function is used to introduce a new order into the market. When traders
+decide to buy or sell an asset, they use `place_order` to specify the details of
+their order, the quantity they wish to buy the `taker_asset` for `taker_amount`
+and sell the `maker_asset` for `maker_amount`. This order is then added to the
+order book, where it waits to be matched by taker(s).
+
+It is important to mention that the users of the Zeitgeist order book do specify
+the price of the assets implicitly. This means that the `place_order` does not
+take a price as an argument, but rather the `taker_amount` and `maker_amount`.
+This means the caller of the `place_order` function does willingly sell the
+`maker_amount` of the `maker_asset` to buy the given `taker_amount` of the
+`taker_asset`. After the execution of `place_order` the `maker_amount` of the
+`maker_asset` is reserved for the order. This means it is not available for the
+user to use in other transactions.
+
+##### Price Example
+
+Let's assume you wish to purchase `42` outcome tokens, referred to as
+`taker_asset`. You decide to offer `7` ZTG, your `maker_asset`, for this
+transaction. Essentially, this means you're ready to exchange `7` ZTG for `42`
+outcome tokens. By doing so, you've implicitly set a rate of `6` outcome tokens
+per `1` ZTG (since `42` divided by `7` equals `6`). Therefore, if a taker agrees
+to your proposed rate, they need to supply `42` outcome tokens (or fewer, in the
+case of partial order fulfillment) and, in return, will get `7` ZTG from you (or
+less, depending on the partial fulfillment scenario).
+
+It's important to note that the market creator's fee is always deducted in the
+market's base currency, which in our example is ZTG. So, if the market creator
+charges a `1%` fee, the taker ends up receiving `6.93` ZTG, while the market
+creator gets `0.07` ZTG. However, the `taker_amount` you receive as the maker
+remains unchanged, as no fees are deducted from the outcome token. It might seem
+like the recipient of the base asset (in this case, ZTG) is always the one who
+pays the fee. But this perspective can vary because each order is unique and may
+already factor in the associated fees at the time of its creation, adjusting the
+price accordingly.
+
+### Fill Order
+
+The crux of any trading activity is the completion of trades, which is
+facilitated by the `fill_order` function. This function is triggered when a
+taker matches a given maker order. The `fill_order` function will execute the
+trade, transferring the specified asset between parties at the agreed price, and
+update the order book to reflect the completion of the trade.
+
+The taker has the ability to partially fill the order for a given
+`maker_partial_fill` amount. The maker always wants the `taker_amount` of the
+specified taker asset (in `place_order`) getting filled by the taker(s). Thus,
+the `maker_partial_fill` (if a partial fill is wanted) should be smaller than
+the `taker_amount` of the maker order.
+
+### Remove Order
+
+Orders may need to be withdrawn from the order book for various reasons, such as
+a change in market strategy or a mistake in the order's details. The
+`remove_order` function allows traders or the system to cancel an existing
+order. This function ensures that the order book remains up-to-date with only
+active intents to trade, thereby preserving market liquidity and participant
+interest.
+
+### Market Creator Fees
+
+As other trading mechanisms on Zeitgeist, the order book does also charge fees
+for the market creator. The fees are charged for each taken order in the
+market's base asset (collateral). As usual, the fee is specified by the market
+creator.

sidebars.js

@@ -28,6 +28,7 @@ const sidebars = {
         "learn/market-rules",
         "learn/governance",
         "learn/court",
+        "learn/order-book",
         "learn/futarchy",
         "learn/comparisons",
         {