ADD in-code documentation integration files.

Includes an explanation of how the in-code documentation is integrated
using Doxygen / Breathe and the raw generated output from the code.

These files are needed to understand how the in-code comments flow into
the documentation and to be able to display and link to those comments
from the documentation.

Change-Id: I275f8c32cfe889da43b4db5b8037e52876ae3c68
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

doc/doxygen/doxygen.rst

@@ -0,0 +1,26 @@
+The In-Code Documentation Generation Process
+############################################
+
+This is the documentation automatically extracted from the code. Doxygen
+generates a huge XML tree in :file:`../xml` that the :program:`Breathe`
+extension imports into Sphinx.
+
+It is so huge though, that, when the full thing is put in, it can take
+ten minutes and plenty of memory to run. For now, it only includes the
+files in :file:`include/microkernel` and :file:`kernel/common`; other
+folders can be added as needed.
+
+The Doxygen pass is independent of the Sphinx pass, but the interesting
+part of linking them toghether is that using Breathe, we can reference
+the code in the documentation and viceversa.
+
+For example, :cpp:type:`K_COMM`. :cpp:class:`k_timer` has been modified
+to have more stuff, visit it's documentation.
+
+This would be an example of referencing function
+:c:func:`__k_memcpy_s()`.
+
+.. toctree::
+   :maxdepth: 2
+
+   doxygen_output.rst

doc/doxygen/doxygen_output.rst

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+.. _code:
+
+
+In-code Documentation
+#####################
+
+.. doxygenindex::
+   :project: Tiny Mountain