docs: Add a document on repository management

Every merge window seems to involve at least one episode where subsystem
maintainers don't manage their trees as Linus would like.  Document the
expectations so that at least he has something to point people to.
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/maintainer/index.rst

@@ -10,5 +10,6 @@ additions to this manual.
    :maxdepth: 2
 
    configure-git
+   rebasing-and-merging
    pull-requests