Run notebooks on RTD with sphinx-gallery and add WorkGraph tutorials

[agoscinski]

Similar as in aiida-workgraph https://github.com/aiidateam/aiida-workgraph/pull/234 I would like to integrate an execution of the notebooks as the notebooks become quickly outdated otherwise. This simplifies the update process of the notebooks as they will be automatically run with each CI, so errors can be immediately seen and then fixed. Since we already adapted the tutorials from the aiida-workgraph to the sphinx-scripts and I would like to move these tutorials out of aiida-workgraph to this repo as fast as possible I focused on moving these as a first example. Nevertheless I would like to adapt all tutorials to be executable in the future.

I tried myst_nb for the execution as it was already provided by the repo, but I ran into issues that ran explained below, so I used the same strategy as in aiida-workgraph by using sphinx-gallery.

Note that the sections Writing workflows and Writing workflows with WorkGraph (the new section including the WorkGraph) have overlaps that could be better resolved, but I did not want to spend the time in the first PR setting up the project structure, since I feel like this could become very time consuming.

Also Note that the CI link checker has been merged to RTD. Since for link checking one also needs to run the tutorials, I thought this is less maintenance as one just needs to update the RTD setup. This also does not affect the building time, because on the second build when creating the html files, the files from the first build can be Reused. Compare the times of https://readthedocs.org/projects/aiida-tutorials/builds/25605020/ of two builds (linkcheck plus html) and https://readthedocs.org/projects/aiida-tutorials/builds/25603891/ one build (only html)