Merge branch 'amiranda/159-generate-doxygen-documentation' into 'master'

Resolve "Automatically generate GekkoFS documentation"

This MR adds a `docs` target to the build system that can be used to automatically generate Sphinx documentation based on ReStructured Text for the project. Additionally, it also processes the source files through Doxygen to generate intermediate XML that can be passed to Sphinx extensions Breathe and Exhale so that GekkoFS classes and functions can be referenced from the Sphinx documentation.

The CI script is also extended to build this documentation and deploy it to the project's site.

Closes #159

See merge request !95