Problem this PR addresses:

Currently, our documentation is residing at three different locations:

• We have the Doxygen documentation directly in our repository. This is our most up-to-date documentation. However, it requires our users to build it themselves using a local copy and Doxygen.
• We do have an online version for our Doxygen documentation at https://sgpp.sparsegrids.org/docs/, but it is horribly out of date (2019) and currently features not automatic updates.
• We have some documentation in our Wiki (https://github.com/SGpp/SGpp/wiki). This documentation includes usage examples (copied from our doxygen), as well as build instruction that can only be found in the Wiki so far (so it is not entirely duplicated). Worse, the math symbols got messed up in the tutorials (see for example https://github.com/SGpp/SGpp/wiki/Base-quick-start-(Python) ).

This is hardly ideal and, in the case of missing math symbols and the out-of-date documentation, rather confusing.

Solution:

I think it is best if we unify these three locations using GitHub pages and our existing Doxygen setup within the code.
This PR adds a new CI pipeline that builds the current Doxygen documentation and pushes it to https://sgpp.github.io/SGpp/ for every commit to master. This way, the documentation is always up-to-date (and the math symbols are displayed correctly).

Next Steps:

I think the next steps should be to move the extra build information from the Wiki to the Doxygen documentation so we have everything in one place. This is also a good opportunity to add build instructions for using our Spack package. I also think we should remove the obsolete documentation from https://sgpp.sparsegrids.org and rather use this site for sgpp-related news, blog posts and publications (not for the code/build documentation).