document how to do flathub releases

[barbeque-squared]

#802 linked to https://flathub.org/apps/eu.usdx.UltraStarDeluxe but I noticed that that still has 2023.12.0 as the most recent release.
Are the steps in https://github.com/UltraStar-Deluxe/USDX/blob/master/RELEASING.md no longer correct? I see there's also https://github.com/flathub/eu.usdx.UltraStarDeluxe but that's not referenced in the release steps at all.

@s09bQ5 considering pretty much the entire history there is you, how does this work? Can anyone make PR's there or is it only certain people? Who can merge, and does it get merged as soon as it looks like some kind of pipeline completes, or does there need to be some kind of review? Can we add random .md files in that repo?
(offtopic for that last one: I'd like to add a readme (or releasing.md) to the website repo as well, because even if it's only a few files, I always find myself editing the wrong one or almost forgetting to update a link somewhere)

From the USDX side I'll probably move the last line of the release steps ("Don't forget to also create a PR for this release in...") to a separate line below all the other release steps, and then have each repo that also needs a PR be its own bullet point in a second list.

EDIT: usecase for wanting release stuff to be pretty well-documented: when I (or anyone) am doing a release, I already need to double-check enough version numbers as it is. I don't want to also need to think about how I should update some file, even if it's near self-explanatory.