Add note about the origin of public skymaps

Specifically, writing about the number of skymaps per event and their meaning - two coming from the preliminary notices, why they might be different, and the later one.

What I wrote comes from the answer to my question on the EMFollow channel by @peter-shawhan, @geoffrey.mo, @deep.chatterjee.

I believe this is not fully spelled out anywhere else, but even if it could be pieced together the page to which I added the note is the one where, as a user, I would expect to find this information.

The pipelines for the userguide are currently failing - for one, "skymaps" is not an English word apparently, and indeed I don't see it being used elsewhere in the user guide. Is it jargon? Should it not be used?

Some more errors are also appearing but I think they're not my fault - looking at the PDF output the links and formatting in the note I added seem to be working.