Updating tutorials format

A few guidelines to update the existing tutorials according to what was discussed in previous meetings:

• Tutorials can have different levels of difficulty within themselves.
• Different parts should be tagged differently, according to the difficulty.
• Tutorials should show the results, hidden under some sort of drop-down.
• Try to add buttons to "skip explanations" or "jump to tutorial/practical".
• Add download links for input files, or embed them into the tutorial page.
• Update all pseudopotential files to those provided by https://www.pseudo-dojo.org/ (PSML).

Some useful tools come from:

• Sphinx design extension: https://sphinx-design.readthedocs.io/en/rtd-theme/index.html
• Sphinx lesson extension: https://coderefinery.github.io/sphinx-lesson/sample-episode-rst/