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/git.md

@@ -0,0 +1,10 @@
+# Git
+
+Zed currently supports the following Git features:
+
+- Diff indicators in buffers and editor scrollbars
+- Git status in the project panel
+- Branch creating and switching
+- Git blame viewing
+
+More advanced Git features—like staging and committing changes or viewing history within Zed—will be coming in the future.