docs: add blurb about target audience to maintainer-profile

It's good to be clear about who the intended target audience for any
given piece of documentation is, as this will help us put new text in
the correct place. Let's encourage submitters to state it explicitly
rather than relying on where they placed it in the directory hierarchy
as there isn't necessarily a one-to-one correspondence between them.

Target audience: documentation contributors and reviewers.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240111094838.3695697-1-vegard.nossum@oracle.com