Build: Add a makefile target for Doxygen documentation

You can now build the doxygen documentation with `make docs` and clean it with `make clean-docs`.

Fixes: #11949

doc/developer-notes.md

@@ -132,6 +132,8 @@ Not OK (used plenty in the current source, but not picked up):
 A full list of comment syntaxes picked up by doxygen can be found at http://www.stack.nl/~dimitri/doxygen/manual/docblocks.html,
 but if possible use one of the above styles.
 
+Documentation can be generated with `make docs` and cleaned up with `make clean-docs`.
+
 Development tips and tricks
 ---------------------------