ci: don't compile whole project when testing docs with Meson

ci: don't compile whole project when testing docs with Meson

Our "documentation" CI jobs, unsurprisingly, performs a couple of tests
on our documentation. The job knows to not only test the documentation
generated by our Makefile, but also by Meson.

In the latter case with Meson we end up building the whole project,
including all of the binaries. This is of course quite excessive and a
waste of compute cycles, as we don't care about these binaries at all.

Fix this by using the new "docs" target that we introduced in the
preceding commit.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>