docs: Add Git page (#14582)

This PR adds a basic Git page to the docs so that we have somewhere to
link to from the Git upsell within Zed.

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -20,6 +20,7 @@
 - [Assistant Panel](./assistant-panel.md)
 - [Channels](./channels.md)
 - [Collaboration](./collaboration.md)
+- [Git](./git.md)
 - [Tasks](./tasks.md)
 - [Remote Development](./remote-development.md)
 - [REPL](./repl.md)