chore: moving notebooks to jupytext and cleaning up workflows

[andreatgretel]

This PR:

1. removes the docs/notebooks folder, and adds docs/notebook_source; files inside this folder are not .ipynb but .py. The docs/notebooks folder can be recreated (exactly the same as before) by running make convert-execute-notebooks, which relies on jupytext to convert/run the notebooks (this was mostly done by @johnnygreco). NB, API keys must be set in order for this command to succeed (should we maybe add a check on the Makefile?)
2. adds a .github/workflows/build-notebooks.yml workflow that runs this make command on the CI, using an API key from Secrets (secrets.NVIDIA_API_KEY, created in my Build account, expires in 12 months)
3. modifies the build-docs.yml and pack-tutorials.yml workflows, which need to call build-notebooks.yml first in order to work (a modified version of build-docs has been tested here)
4. small cleanups on existing workflows - standardized names, added more details for steps

[johnnygreco]

This is awesome @andreatgretel, thank you 🙏

[nabinchha]

🚢 Thanks @andreatgretel!