docs: Add docs for defining slash commands in extensions (#16303)

This PR adds docs for defining slash commands within extensions.

Release Notes:

- N/A

docs/src/extensions/developing-extensions.md

@@ -6,6 +6,7 @@ Extensions can add the following capabilities to Zed:
 
 - [Languages](./languages.md)
 - [Themes](./themes.md)
+- [Slash Commands](./slash-commands.md)
 
 ## Directory Structure of a Zed Extension
 
@@ -22,10 +23,6 @@ description = "My cool extension"
 repository = "https://github.com/your-name/my-zed-extension"
 ```
 
-<!--
-TBD: Document `slash_commands`, `indexed_docs_providers` (see: extensions/gleam/extension.toml)
--->
-
 In addition to this, there are several other optional files and directories that can be used to add functionality to a Zed extension. An example directory structure of an extension that provides all capabilities is as follows:
 
 ```