Publish Doxygen docs using Github pages

[pjattke]

As described in this blogpost, it's possible to publish the code docs generated by Doxygen using Github pages. I prepared the necessary files (see a39ce2d). All that is now still required is to enable Github pages for this project. This requires repository owner permissions.

A future task could be to use Github Actions to automatically build the Doxygen docs and deploy them to Github pages.

[AlexanderViand]

Now available at https://marblehe.github.io/AST-Optimizer/html/index.html
Keep in mind that the repo is private, but the github pages are public (should be fine for now, though)