meson: install static files for HTML documentation

Now that we generate man pages, articles and user manual with Meson the
only thing that is still missing in an installation of HTML documents is
a couple of static files. Wire these up to finalize Meson's support for
generating HTML documentation.

Diffing an installation that uses our Makefile with an installation that
uses Meson only surfaces a couple of discepancies now:

- Meson doesn't install "everyday.html" and "git-remote-helpers.html".
These files are marked as obsolete and don't contain any useful
information anymore: they simply point to their modern equivalents.

- Meson doesn't install "*.txt" files when asking for HTML docs. I'm
not sure why our Makefiles do this in the first place, and it does
seem like the resulting installation is fully functional even
without those files.

Other than that, both layout and file contents are the exact same.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>