Improvement: Move wget of Doxygen version to Docker image rather than Github workflow

[spenke91]

Downloading the specific Doxygen version via wget in the workflow t8code_doxygen_check / check_documentation is suprisingly unstable and causes a lot of unnecessary CI fails.

I just realized it may be easiest to just download the version in our t8code containers already. I will do so within the next days.