Cross-link docs from sphinx to doxygen
Remove doc/source/user_manual/reference.rst (was empty)
Bundle rawfiles Sphinx directive for docs build
Move doc/html/doc to doc/source/fhelp
Add tools_reference.rst page
Move images from doc/html/images to doc/source/_static
Move images from test to doc/source/_static
Move ctools.css file from doc/html to doc/source/fhelp
Remove old HTML docs pages that have been transferred to RST
Add doc/html to .gitignore

The html output folder should only contain generated files,
so that it becomes possible to clean the docs build via
`rm -r doc/html`.
Add intersphinx_mapping to gammalib
Merge branch 'doxylink' into integration