Restructure and small fixes

[leburgel]

Some reorganization and small fixes here and there. Broad summary of the changes:

• Added a 'Preliminaries' section containing the intro to software and some (very basic) links to other useful resources. This seemed better than having it in the more physics-oriented introduction.
• Split the introduction to quantum many body physics into separate sections to improve overall readability.
• Split off section on tensors and tensor networks after the general introduction. This is mostly the same, except that I moved up the part on tensor network states since this didn't really fit under the MPS banner in the first place.
• Some bug and test fixes in the example code.

[github-actions]

PR Preview Action v1.4.6
🚀 Deployed preview to https://quantumghent.github.io/TensorTutorials/pr-preview/pr-26/
on branch gh-pages at 2024-02-02 10:05 UTC

[lkdvos]

Looks great!
I think backups can be safely deleted, if ever needed, we can find them in the github history.