meson: print docs backend as part of the summary

meson: print docs backend as part of the summary

Our documentation can be built with either Asciidoc or Asciidoctor as
backend. When Meson is configured to build documentation, then it will
automatically detect which of these tools is available and use them.
It's not obvious to the user though which of these backends is used
unless the user explicitly asks for one backend via `-Ddocs_backend=`.

Improve the status quo by printing the docs backend as part of the
"backends" summary.

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