meson: wire up support for AsciiDoctor

meson: wire up support for AsciiDoctor

While our Makefile supports both Asciidoc and AsciiDoctor, our Meson
build instructions only support the former. Wire up support for the
latter, as well.

Our Makefile always favors Asciidoc, but Meson will automatically figure
out which of both to use based on whether they are installed or not. To
keep compatibility with our Makefile it favors Asciidoc over Asciidoctor
in case both are available.

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