meson: introduce a "docs" alias to compile documentation only

meson: introduce a "docs" alias to compile documentation only

Meson does not currently provide a target to compile documentation,
only. Instead, users needs to compile the whole project, which may be
way more than they really intend to do.

Introduce a new "docs" alias to plug this gap. This alias can be invoked
e.g. with `meson compile docs`.

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