Add a CMakeLists.txt which provides the 'manual' target for building the Sphinx documentation. This may require you to set the SPHINXBUILD_EXECUTABLE CMake variable to the sphinx-build executable

Change the file extension to 'rst' rather than 'txt' to avoid clashes with CMakeLists.txt and to help syntax highlighting.

Add the doxylink Sphinx extension from sphinx-contrib to provide Doxygen links.