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>

meson.build

@@ -2099,11 +2099,18 @@ endif
 
 subdir('bin-wrappers')
 if get_option('docs') != []
+  doc_targets = []
   subdir('Documentation')
 endif
 
 subdir('contrib')
 
+# Note that the target is intentionally configured after including the
+# 'contrib' directory, as some tool there also have their own manpages.
+if get_option('docs') != []
+  alias_target('docs', doc_targets)
+endif
+
 exclude_from_check_headers = [
   'compat/',
   'unicode-width.h',