doc: add Doxygen configuration

Add a simple Doxygen configuration that allows to build documentation
for the application APIs found under include/.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

.gitignore

@@ -11,3 +11,6 @@
 /twister-out*
 
 __pycache__/
+
+# docs
+/doc/_build*