meson: properly wire up dependencies for our docs

meson: properly wire up dependencies for our docs

A couple of Meson documentation targets use `meson.current_source_dir()`
to resolve inputs. This has the downside that it does not automagically
make Meson track these inputs as a dependency. After all, string
arguments really can be anything, even if they happen to match an actual
filesystem path.

Adapt these build targets to instead use inputs.

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