CI: rework workflow to leverage caching

[SMoraisAnsys]

In order to shorten the CI/CD process when including new examples, it could be interesting rework the way we are doing things by building the documentation while only executing the new example(s).

Idea of how to:

• cache previous examples
• use nbsphinx_execute="auto"

Not sure that we should go with that approach yet. We should wait to see how this repo evolves before doing anything as proposed.

Another solution of CI/CD process could be to add a job to create a sub documentation containing only the new examples and to proceed with the full documentation if this job works.