docs: Add doc about creating new endpoints (#645)

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## Changes

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docs/getting_started/ADDING_AN_ENDPOINT.md

@@ -0,0 +1,96 @@
+# Adding a new endpoint
+
+## Adding a new endpoint to Rivet Core
+
+1. Update the respective `__package.yml__` file with the new endpoint you want
+   to add. This may require adding something to the `common.yml` file that
+   should be at `../common.yml` relative to the package file.
+
+1. Run the script `./scripts/fern/gen.sh`. Currently that might look something
+   like:
+
+    `FERN_REPO_PATH=/home/forest/data/git/fern ./scripts/fern/gen.sh`
+
+    This will generate new files in `sdks/`. We'll mostly care about the new Rust
+    files in `sdks/rust/full/rust/src/...`.
+
+1. If you added any enums or structs to the `common.yml` file, you'll need to
+   add/update the impls in `lib/convert/src/impls`. This allows enums to be
+   mapped from the generated API type to an internal mapping. For example, in
+   `lib/convert/src/impls/admin.rs`:
+
+    ```rust
+    impl ApiFrom<models::AdminProvider> for backend::cluster::Provider {
+       fn api_from(value: models::AdminProvider) -> backend::cluster::Provider {
+          match value {
+             models::AdminProvider::Linode => backend::cluster::Provider::Linode,
+          }
+       }
+    }
+    ```
+
+1. Add the api endpoint in the related route file for the api. Say
+   `svc/api/admin/src/route/mod.rs`:
+
+    ```rust
+    define_router! {
+        routes: {
+            "cluster": {
+                POST: cluster::create(
+                    body: models::AdminClusterCreateRequest,
+                ),
+            },
+        },
+    }
+    ```
+
+1. Create the handler for the new endpoint in the related handler file. Say
+   `svc/api/admin/src/handler/cluster.rs`:
+
+    ```rust
+    pub async fn create(
+        ctx: Ctx<Auth>,
+        body: models::AdminClusterCreateRequest,
+    ) -> GlobalResult<models::AdminClusterCreateResponse> {
+        // Your handler code here
+    }
+    ```
+
+    Some tips for this:
+
+    - Mapping from an enum in the model to an integer in the protobuf-generated
+      model can be done with `as i32` (or conversely `from_i32`)
+
+## Connecting to the endpoint with Bolt
+
+If Bolt needs a new CLI command that will interface with this api, these are the
+steps you'll need to take:
+
+1. Add the new command to the respective existing or new bolt command.
+
+1. Openapi requests are generated for each endpoint defined in the
+   `__package.yml__` file. Here's an example of calling one of these requests:
+
+    _Note: These current docs only make use of the cloud token, hub and cli
+    endpoint docs to come later._
+
+    ```rust
+    admin_clusters_api::admin_clusters_create(
+       &ctx.openapi_config_cloud().await?,
+       models::AdminClustersCreateRequest {
+          name_id: cluster_name_id,
+          owner_team_id: Some(Uuid::parse_str(&owner_team_id).unwrap()),
+       },
+    )
+    .await?;
+    ```
+
+    This also makes use of the same `models` that are generated by the Openapi,
+    allowing ser/de data to be passed across the wire.
+
+    Some tips for this:
+
+    - In some cases, you might need to write a custom request instead of using a
+      generated one. For example, Fern allows get requests to `allow-multiple` of
+      the same repeated parameter. The generated request does not interpret this
+      correctly, so you'll need to write a custom request for this case.