ci: write a workflow that reminds us to rebuild the manual before releasing

[sbillinge]

Problem

There is a "manual" step to rebuild the tutorial manual in PDFgui. We don't want to forget to do that in future releases.

Proposed solution

Per conversations with @Tieqiong it is a bit complicated, and bespoke to this project, and so almost certainly not worth writing up an automated workflow for doing this. So maybe just write a workflow that sends a warning somehow (tbd) to the person releasing the code to do it, along with detailed instructions for doing it (so they don't have to look it up). @sbillinge would be very happy with that solution. The most important think here is just to not forget to do it.