adjust the links and information regarding where to get help

Summary

Remove reference to mailing list as mentioned in #303.

Update to current discussion medium.

[x] Bug fix (non-breaking change which fixes an issue).
[ ] New feature (non-breaking change which adds functionality).
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

[x] It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
[x] I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
[ ] I've added tests for my code.
[x] I have verified whether my change requires changes to the documentation
[x] My change either requires no documentation change or I've updated the documentation accordingly.
[x] My branch has been rebased to main, keeping only relevant commits.