Merge branch 'fp' into docs-mw

The top-level index.rst file is the entry point for the kernel's
documentation, especially for readers of the HTML output.  It is currently
a mess containing everything we thought to throw in there.  Firefox says it
would require 26 pages of paper to print it.  That is not a user-friendly
introduction.

This series aims to improve our documentation entry point with a focus on
rewriting index.rst.  The result is, IMO, simpler and more approachable.
For anybody who wants to see the rendered results without building the
docs, have a look at:

  https://static.lwn.net/kerneldoc/

This time around I've rendered the pages using the "Read The Docs" theme,
since that's what everybody will get by default.  That theme ignores the
directives regarding the left column, so the results are not as good there.
I have a series proposing a default-theme change in the works, but that's a
separate topic.

This is only a beginning; I think this kind of organizational effort has to
be pushed down into the lower layers of the docs tree itself.  But one has
to start somewhere.