Add Zstandard source link

[emmatyping]

One tricky thing with the zstd sources is that they include Github Actions workflow definitions. I'm not sure if that can be imported as-is without causing actions to start triggering.

[github-actions]

We do not accept PRs on this repository. Please file an issue at https://github.com/python/cpython requesting an update to the source packages in this repository.

[gpshead]

for the GH actions configs, if those existing within a branch cause issues it is reasonable for us to strip those out. for release management I feel like we've got code release managers use elsewhere (they'll know, i don't recall where off the top of my head as a non RM) that automates fetching relevant specific third party sources for the release build.