assistant: Show a message when no docs providers are available (#14207)

This PR updates the `/docs` slash command to show a message to more
clearly indicate when there are no available docs providers.

<img width="379" alt="Screenshot 2024-07-11 at 10 31 53 AM"
src="https://github.com/zed-industries/zed/assets/1486634/d079f87c-4933-4da9-ad82-34dbfe6a284c">

Release Notes:

- N/A