Workflow updates

[christian512]

This updates the GitHub Actions workflow for the tutorials. I have added:

• Conda environment caching
• Use mamba instead of conda
• automatic link verification in markdown files
• separate folders for notebooks with qutip 4 and qutip 5
• testing and publishing of notebooks with particular qutip version

[hodgestar]

Hmm. The build on the main branch died. Maybe there is some sort of race condition somewhere?

[christian512]

The error occurred on testing one notebook, which worked in the build for the PR. I just restarted, to see if the error persists.