ci: perform build and smoke tests for Meson docs

ci: perform build and smoke tests for Meson docs

Our "documentation" CI job performs a couple of tests against our
documentation. Part of these tests is to check whether documentation
builds at all and whether it spits out the expected set of files. We
don't yet have such a test for Meson, which means that we wouldn't
notice at all if building the documentation were to break. As a result,
breakages as fixed by 87eccc3a81d (meson: fix building technical and
howto docs, 2025-03-02) are easy to go unnoticed.

Address this test gap by starting to build both manpages and HTML sites
as part of the CI job.

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