Replace tool HTML help with Sphinx-generated docs

Replace tool HTML help with Sphinx-generated docs

Instead of generating HTML pages directly, make 'gmx help -export rst'
export reStructuredText that is used as input for Sphinx to generate the
help pages as part of the Sphinx documentation.  Make other docs refer
to these with a native Sphinx construct :ref:`gmx something`, as it
seems not possible to make `gmx something`_ really work.
:doc:`/programs/gmx-something` works even nicer, but is a bit too much
typing.

Make various minor corrections to existing help documentation so that
they can be converted to rst without errors; the output is still very
ugly in several places (but it wasn't any better in the old HTML...).
This change does not aim to fix all problems; it will probably be easier
once we have more freedom in using different rst constructs.
Links to file formats do not work, but those should be easier to fix
once the file format descriptions are also in rst.

Remove installation of the old share/html/ pages, as there is very
little stuff left there.

Change-Id: Ie9d7f1435656d9cf1f60a7e1c4c3c2e440af227d