Add FAQ entry about theme overrides

[frederik-elwert]

Currently, the documentation refers to creating new themes a lot. It is much less apparent how one can customize an existing theme by overriding individual templates or adding new ones. This commit adds an FAQ for this, and also mentions the mechanism in the FAQ on custom templates.

Pull Request Checklist

• [x] Ensured tests pass and (if applicable) updated functional test output
• [ ] Conformed to code style guidelines by running appropriate linting tools
• [ ] Added tests for changed code
• [ ] Updated documentation for changed code

[frederik-elwert]

I wanted to submit this before a busy day gets the best of me. But I should probably add: This was something that I felt was a bit lacking in the docs. Other people might feel differently, so I'm completely fine if this suggestion gets rejected.