doc/api/libbabeltrace2/DoxygenLayout.xml: use `topics` tab

doc/api/libbabeltrace2/DoxygenLayout.xml: use `topics` tab

It looks like the Doxygen project decided [1] to use the "topic"
terminology instead of "module" to avoid confusion with C++20 modules.

Add a `topics` tab to `DoxygenLayout.xml` because otherwise that
navigation tab won't show up.

Leaving `modules` for older Doxygen versions.

Also changing all the "module" terms to "API"/"page" in the actual
documentation to completely part from the old "module" Doxygen concept.

[1]: https://github.com/doxygen/doxygen/commit/6d80fc7e5d03c259b1a7280972e0b28884217655

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6f535c42c2bd5f55e5727a3245a2d744b28c187c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12630