Add weekly GitHub Action to build docs whilst re-executing the pre-executed notebooks

[namurphy]

That is one way we could prevent an API call when building the docs! But I want to try and avoid that on principle since pre-executed notebooks are way more likely to get buggy/out of date.

Originally posted by @pheuer in #2570 (comment)

Following up on the comment by @pheuer, we could probably add a GitHub Action that would do a full documentation build and execute all notebooks. We currently pre-execute some of the synthetic charged particle radiography notebooks, but these can get buggy or out-of-date (as pointed out above). Having a cron job would let us figure out when things break, in particular before releases.