Add some pages for C++ resources.

[drwells]

This is a common problem we all have and it would help a lot to have a single place to store a curated list - see also dealii/dealii#17483

[marcfehling]

Maybe having some additional git resources would be nice to have as well.

[jpthiele]

In terms of git these two ressources are good and both initiatives offer regular workshops
https://swcarpentry.github.io/git-novice/
https://coderefinery.github.io/git-intro/

and maybe as a general suggestion we could add ressources/links for (research) software engineering too.