Currently there's no document explaining how idmappings work at all.
Add a document that gives an introduction and also goes into a bit more
detail for more advanced use-cases.

Link: https://lore.kernel.org/r/20210727104416.828293-1-brauner@kernel.org
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>