DOCS: Skip execution of some examples

[SMoraisAnsys]

This PR reworks the way the doc was previously created by adding a few steps.

The main change is that we are manually using jupytext to create the notebooks while we were delegating that previously.
While see could seem like a trivial change, it provides us the opportunity to then extend the metadata associated to the notebook while previously it was executed on the fly without us being able to add metadata.

Now, after the creation of the notebook, we disable some examples execution by adding this to the notebook’s JSON metadata:

"nbsphinx": {
  "execute": "never"
},

---

Note: Now that we have something working, it might have been possible to simply add a unique event hook at readtime to add the previously mentioned metadata. However, I'm not sure to how easy this would be as we are not directly the one "playing" with the file content. I might rework that in a future PR to simplify things.

[Samuelopez-ansys]

Great!