Merge branch 'alabaster-rb' into docs-mw

For a long time we have rejoiced that our HTML output from Sphinx is far
better than what we got from the old DocBook toolchain.  But it still
leaves a lot to be desired; the following is an attempt to improve the
situation somewhat.

Sphinx has a theming mechanism for HTML rendering.  Since the kernel's
adoption of Sphinx, we have been using the "Read The Docs" theme — a choice
made in a bit of a hurry to have *something* while figuring out the rest.
RTD is OK, but it is not hugely attractive, requires the installation of an
extra package, and does not observe all of the Sphinx configuration
parameters.  Among other things, that makes it hard to put reasonable
contents into the left column in the HTML output.

The Alabaster theme is the default for Sphinx installations, and is bundled
with Sphinx itself.  It has (IMO) nicer output and gives us the control
that we need.

So: switch to Alabaster.  Additional patches adjust the documentation and
remove the RTD references from scripts/sphinx-pre-install.

The penultimate patch changes the way that kerneldoc declarations are
rendered to (IMO) improve readability.  That requires some changes to
kernel-doc to output a new container block and some CSS tweaks to improve
things overall.

It should be noted that I have a long history of inflicting ugly web
designs on the net; this work is a start, but I think we could do far
better yet.  It would be great if somebody who actually enjoys working with
CSS and such would help to improve what we have.