Skip building example notebooks in CI via GitHub Actions

[namurphy]

PlasmaPy currently performs two doc builds in CI. One is with Read the Docs which includes a full doc preview, and the other is with GitHub Actions. I've been working to speed up the doc build done by GitHub Actions. It's down to ∼4 min!

I did some benchmarks in #2562. Skipping the execution of example notebooks would reduce doc build times by ∼30 s (and would be longer as we add more notebooks). Given how important rapid feedback is to CI, I'm currently thinking that it would be worth it to skip execution of notebooks in the GitHub Action. The RTD build would continue to execute notebooks.