Complete release automation / make a release

[fingolfin]

1. import the new scripts into the GAP repository (see PR Add scripts for making releases / distributions #4279)
2. finish the release notes... manually, or with a script, or ... see issue Simplify the creation of release notes #4257 and PR Python script to get an overview of changes with PyGithub #4277
3. set up a GitHub Action triggered by a cron time for "nightlies", and one triggered by new tags (whether they share code or are really two fully separate actions: I don't care) (see PR GH Actions: automate release tarball creation (WIP) #4282); @FriedrichRober and @ssiccha wrote the code for that, and I've been tweaking it a bit. I'll open a PR for it, but want to first tweak it a bit more, so
4. allow updaing manuals on the website
   • add code to extract the GAP & package manuals from a release tarball (work begun here: WIP add helper script to copy package manuals GapWWW#227)
   • Decide where/how manuals for GAP and packages are hosted in the future (see Decide where/how manuals for GAP and packages are hosted in the future GapWWW#223)
   • unless we resolve the preceding point quickly, for now insert a manual step into the release intructions ("... now use the following commands to copy the files you got via method X to serve Y...")
     make sure that the website updates work
5. finish the rest of the website related work
6. also set up GitHub Action automation on the GapWWW repository doing the above (perhaps with a manual trigger?)
7. get @ChrisJefferson "new Cygwin release/installer" working, and ideally also have a GitHub Action job for this which spits out the .exe installer at the end, ready for use on the website...
8. test the whole setup...
9. make an actual release

Did I forget something? Please feel free to edit this text!

[fingolfin]

We made two releases with the system now, it works