Migrate to pixi for development

[DunklesArchipel]

While looking into #56 and #55 I tried to fix this by migrating to jupyter book V2 and implementing a more robust development environment (pixi). I added usage instructions to contributing.md and installation instructions to the readme. The dependencies are located in the pixi.toml file. Pixi would also accept a pyproject.toml, which includes pixi as a tool.

Since this is rather experimental, I keep this as a draft.

• The new look is less compact than the old version (simply run pixi run serve to have a look). I liked the compact version.

Following a few things that I could not address and some suggestions, that can also be transferred into an issue.

• I could not add jcamp as a dependency. Solving the environment fails. I tried it in another repository and had the same issue. Not sure if it is a platform or python version issue. This and also CAS Common Chemistry Needed update #54 compromise notebook execution (see next)
• Preferably the ipynb's are executed during the build and the build should fail when an error is raised in a cell. In principle it works by running pixi run execute, but the build fails and does not provide an ambiguous error message. This will help in finding issues faster, since all notebooks will be checked for each pull request (dependabot alert).
• There seem to be broken links, which you one can check via pixi run build.
• Change ipynbs into myst-md notebooks. This has some flexibility in terms of which cells/notebooks should be executed and one can run some stuff in the background in hidden cells. The development process works well with jupytext.
• Binder has not been checked so far.

[stuchalk]

Thanks for your work on this. Sadly, I have not had time to review the PR but will try to do so by the end of this week. Moving to JB2 seems like a good idea, but I need to work out what it solves and what it affects that will also need fixes.