Give more detail on creating customized versions

[jfehrle]

Identical to #430, which was on branch "main" by mistake.

I'm happy to revise.

[jfehrle]

@MSoegtropIMC I appreciate the doc changes but they need to be corrected in a few ways. E.g. it is easy to get a tar ball for any commit or tag without releasing it. One should mention how this works rather than how to make a release in github. For the opam package the main problem is that most packages are in a different repo. Only the coq packages are in the repo you mentioned. As is this is very confusing. But as I said, just remove the doc changes and keep them in the separate PR.

As I recall, it took me at least 6-8 hours and to figure out how to do this stuff with your help over a couple days. It really shouldn't be that hard. Changing the instructions to do something else is another project. If you give me some guidance I'll try to revise it. Otherwise I fumble around with stuff that's not documented, kind of randomly trying one thing after another.

For the download, if I change the url to https://api.github.com/repos/jfehrle/coq/tarball/debug_pl_8_19 the build still seems to work even if I don't update the checksum. If I do a wget with that URL and checksum it, the checksum is different. So not sure what's happening.

For downloading in order to compute the checksum, should I recommend wget? I tried using curl but didn't find the right parameters.

[MSoegtropIMC]

@jfehrle : I can adjust your text and you can review it then. I just wanted it to be out of the Ltac2 debugger PR so that we can do this separately.