api.rst - add title of `API Documentation`

Otherwise `viki.fabric.helpers` gets displayed as the title.

Signed-off-by: Pang Yan Han <yanhan@viki.com>
Add usage guide for the `viki.fabric.git` module

This is a much more complex module compared to `viki.fabric.helpers` and
deserves some good documentation to allow users to have a smoother
experience using it.

Signed-off-by: Pang Yan Han <yanhan@viki.com>