Start section on using Git to work with data repositories

Currently some Git commands are in Advanced topics. Since command line, besides some simple edits in text files, will be the main interface to work with data repositories, lets organize a separate section with Git commands and workflows.

Taks:

• Start a new section in documentation.
• Move Advanced topics to the new section.
• Explain prerequisites.
• Add clone command.
• Add git branch -rv to view remote branches after clone.
• Explain the basic Git workflow.