docs: Bring some order to filesystem documentation

Documentation/filesystems is, like much of the rest of the kernel's
documentation, a jumble of unorganized information.  Split the
documentation into categories and try to bring some order to the top-level
index.rst files.  No text changes other than a few section-introductory
blurbs; this is all just moving stuff around.

Cc: linux-fsdevel@vger.kernel.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>