doc: give a more thorough id handling explanation

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/[email protected]
Cc: Seth Forshee <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Aleksa Sarai <[email protected]>
Cc: [email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: Christian Brauner