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>

.gitignore

@@ -61,7 +61,6 @@ __pycache__
 /src/babeltrace2-ctf-writer.pc
 TAGS
 cscope*
-doc/api/Doxyfile
 *.gcno
 *.gcda
 *.gcov

configure.ac

@@ -563,7 +563,7 @@ AS_IF([test "x$enable_api_doc" = "xyes"],
     DX_XML_FEATURE(OFF)
     DX_PDF_FEATURE(OFF)
     DX_PS_FEATURE(OFF)
-    DX_INIT_DOXYGEN([Babeltrace], [$(srcdir)/Doxyfile], [output])
+    DX_INIT_DOXYGEN([Babeltrace 2], [$(builddir)/Doxyfile], [output])
     AS_IF([test -z "$DX_DOXYGEN"],
 	  [AC_MSG_ERROR([You need doxygen to enable the API documentation])]
     )
@@ -724,8 +724,9 @@ program_transform_name="s&babeltrace2\.bin&babeltrace2&;$program_transform_name"
 AC_SUBST(program_transform_name)
 
 AC_CONFIG_FILES([
-	doc/api/Doxyfile
 	doc/api/Makefile
+	doc/api/libbabeltrace2/Doxyfile
+	doc/api/libbabeltrace2/Makefile
 	doc/bindings/Makefile
 	doc/bindings/python/Makefile
 	doc/contributing-images/Makefile