Add git guide for mathlib4 contributors (#643)

Adds a comprehensive git guide aimed at mathematicians new to git who
want to contribute to mathlib4. Covers forking, cloning, remotes,
branches, PRs, and collaboration workflows.

---------

Co-authored-by: Johan Commelin <johan@commelin.net>
Co-authored-by: Robin Carlier <57142648+robin-carlier@users.noreply.github.com>
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>

Author: 4 people

data/menus.yaml

@@ -50,11 +50,11 @@
 - title: Contributing
   items:
     - Pull request lifecycle: contribute/index.html
+    - Git Guide for Mathlib4 Contributors: contribute/git.html
     - Naming conventions: contribute/naming.html
     - Code style guideline: contribute/style.html
     - Documentation style: contribute/doc.html
     - Commit conventions: contribute/commit.html
     - Pull request review guide: contribute/pr-review.html
     - Contribution statistics: mathlib_stats.html
     - Tags and branches: contribute/tags_and_branches.html
-