Document libbabeltrace2's C API

This patch adds initial documentation for the Babeltrace 2 library's
C API using Doxygen.

The Doxygen project is located in `doc/api/libbabeltrace2`, as we can
eventually add `doc/api/libbabeltrace2-ctf-writer`.

To be able to use Doxygen's member grouping [1], I had to join some
header files (`const` and non `const` headers, for example), because
otherwise I could not get some functions in separate files to be in the
same member group in the order I want. In the end, the library user
includes `<babeltrace2/babeltrace.h>`, so how we organize the headers
exactly is not so crucial.

[1]: http://www.doxygen.nl/manual/grouping.html#memgroup

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6d1dc2e7c5ee63fcd4220d0fd9f0931d361d2f31
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2807
Tested-by: jenkins <jenkins@lttng.org>