Add design tweaks to the AI configuration panel (#15894)

This PR polishes elements around setting up LLM providers on the
Assistant panel, including:

- [x] Adding banners for promoting Zed AI and to deal with the "No
provider set up" scenario
- [x] Tweaking the error popover whenever there's no API key added
- [ ] Making configuration panel scrollable

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Release Notes:

- N/A

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Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>