git: Add an onboarding and banner flow (#26518)

TODO:

- [ ] Hide the reset onboarding action (only useful for development,
uncomment:
https://github.com/zed-industries/zed/pull/26518/files#diff-f0ce01d9a3df30f60c64b6f9906c54aa0191246a58dbf5297ee321575a180879R96)
- [x] Get a designer to replace the modal background (@danilo-leal)

Release Notes:

- Added a small onboarding banner for the git launch

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>