This PR is a quick-and-dirty approach to clarify what is happening when
a user doesn't have an API key so the assistant opens with the key
editor.

- Adds some text explaining how to close the panel
- Explains that other subscriptions don't cover the assistant, it needs
an api key.
- Helps people understand where to go to get an API key.

This is a Zed 1 change only, a bandaid. Zed 2 will have a better
solution: [Working
issue](https://github.com/zed-industries/zed/issues/3461)

Release Notes:

- N/A