Improve documentation about the different git repos and point to it
    
    This commit improves the documentation about the different git
    repositories associated with a project.
    In addition, we're fixing the indentation in the repo_master template
    and adding a link to the documentation about the different git repos.
    
    Fixes https://pagure.io/pagure/issue/3451
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>