docs: Add doc about creating new endpoints

docs/getting_started/ADDING_AN_ENDPOINT.md

@@ -0,0 +1,72 @@
+# 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.
+
+2. 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/...`.
+
+3. If you added any enums 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,
+         }
+      }
+   }
+   ```
+
+4. 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" / "create": {
+                POST: cluster::create(
+                    body: models::AdminClusterCreateRequest,
+                ),
+            },
+        },
+    }
+    ```
+
+5. 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.
+
+2.