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: [email protected]
Cc: Al Viro <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>

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