docs/devel: Rules on #include in headers

docs/devel: Rules on #include in headers

Rules for headers were proposed a long time ago, and generally liked:

    Message-ID: <[email protected]>
    https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg03345.html

Wortk them into docs/devel/style.rst.

Suggested-by: Bernhard Beschow <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20221222120813[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Bernhard Beschow <[email protected]>