docs: Document context servers (#21170)

This PR adds documentation for context servers.

Release Notes:

- N/A

docs/src/extensions/context-servers.md

@@ -0,0 +1,39 @@
+# Context Servers
+
+Extensions may provide [context servers](../assistant/context-servers.md) for use in the Assistant.
+
+## Example extension
+
+To see a working example of an extension that provides context servers, check out the [`postgres-context-server` extension](https://github.com/zed-extensions/postgres-context-server).
+
+This extension can be [installed as a dev extension](./developing-extensions.html#developing-an-extension-locally) if you want to try it out for yourself.
+
+## Defining context servers
+
+A given extension may provide one or more context servers. Each context server must be registered in the `extension.toml`:
+
+```toml
+[context-servers.my-context-server]
+```
+
+Then, in the Rust code for your extension, implement the `context_server_command` method on your extension:
+
+```rust
+impl zed::Extension for MyExtension {
+    fn context_server_command(
+        &mut self,
+        context_server_id: &ContextServerId,
+        project: &zed::Project,
+    ) -> Result<zed::Command> {
+        Ok(zed::Command {
+            command: get_path_to_context_server_executable()?,
+            args: get_args_for_context_server()?,
+            env: get_env_for_context_server()?,
+        })
+    }
+}
+```
+
+This method should return the command to start up a context server, along with any arguments or environment variables necessary for it to function.
+
+If you need to download the context server from an external source—like GitHub Releases or npm—you can also do this here.

docs/src/extensions/developing-extensions.md

@@ -7,6 +7,7 @@ Extensions can add the following capabilities to Zed:
 - [Languages](./languages.md)
 - [Themes](./themes.md)
 - [Slash Commands](./slash-commands.md)
+- [Context Servers](./context-servers.md)
 
 ## Directory Structure of a Zed Extension