Add table of contents to GitHub release

[olexandr-konovalov]

In addition to #120, another file that would be useful to have is the .md file with the table like https://github.com/gap-system/gap/wiki/Package-updates-status, which will describe package versions exactly in those archives, and will indicate which of them are newer than in the latest GAP release.

[fingolfin]

Even better would be to provide a machine readable package-info.json file via the script https://github.com/gap-system/gap/blob/master/dev/releases/PackageInfos-to-JSON.g (which also produced https://github.com/gap-system/gap/releases/download/v4.11.1/package-infos.json.gz).

The filename could be the same as for the distribution archives, just with .tar.gz replaced by .json (or perhaps .json.gz, as they can be quite hefty).

Also, ideally those files would also be copied to https://github.com/gap-system/gap-distribution/releases/tag/package-archives

Once we have that, we can use them e.g. to produce a large part of the "Updates/new packages" section in CHANGES.md for releases, as part of the script being developed at gap-system/gap#4277