doxygen_to_devhelp: Make the API reference the front page

doxygen_to_devhelp: Make the API reference the front page

The tutorial is not necessarily a great entry point for the libdbus
documentation: it's infrequently updated, and we should probably have
the "If you use this low-level API directly, you're signing up for some
pain" message from the API reference show up in devhelp more immediately.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=106186
Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: Add longer commit message with rationale]
Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit c84ac8b1ef3a9fa18127b2462ead369c4e8846bd)