doc/sphinx: Add 'weston_compositor', 'weston_output' and 'weston_head' API

Demonstrates how to call/use the 'doxygen*' breathe directives. Make
use of previous patches that tag symbols using 'ingroup' doxygen
command.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

doc/sphinx/toc/libweston/compositor.rst

@@ -5,3 +5,9 @@ Compositor
 aggregates all the other objects and maintains their state. You can create it
 using :func:`weston_compositor_create`, while for destroying it and releasing all
 the resources associated with it, you should use :func:`weston_compositor_destroy`.
+
+Compositor API
+--------------
+
+.. doxygengroup:: compositor
+   :content-only:

doc/sphinx/toc/libweston/head.rst

@@ -16,3 +16,9 @@ driving a :type:`weston_head`. :type:`weston_head` should be initialized using
 A :type:`weston_head` must be attached/detached from a :type:`weston_output`.
 To that purpose you can use :func:`weston_output_attach_head`, respectively
 :func:`weston_head_detach`.
+
+Head API
+--------
+
+.. doxygengroup:: head
+   :content-only:

doc/sphinx/toc/libweston/output.rst

@@ -8,3 +8,9 @@ already enabled.
 
 The reverse operation, :func:`weston_output_disable`, should be used when there's
 a need to reconfigure the output or it will be destroyed.
+
+Output API
+----------
+
+.. doxygengroup:: output
+   :content-only: