doc: extensions: add doxybridge

Add an initial version of doxybridge, an extension that allows to use
Sphinx C domain to automatically reference Doxygen pages.

It also introduces minimal support for `.. doxygengroup` directive,
which effectively links to the group's Doxygen page.

Co-authored-by: Benjamin Cabé <benjamin@zephyrproject.org>
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>