CG: Revise Git documentation
This completely reshapes CG section 3, "Working with source code".
- Remove obsolete and duplicate parts. This includes how to deal with git-formatted patches, for example (since our new workflow doesn't use them).
- Reorganise the rest entirely. Make it simpler and more straightforward. Also remove unnecessary technical details (these belong to external documentation).
- Drop the bit of instructions for using git-gui on Windows. A terminal was needed anyway. Instead, link to a list of Git GUIs on the official Git website. This is more likely to make new contributors happy since it seems those GUIs are more complete, elaborate and user-friendly.
The new structure is:
- Setting up
- Git cheat sheet
- Lifecycle of a Merge Request
- Commit messages
- Commit acces
- Further Git documentation resources
- Repository directory structure
- Other repositories