gh-64940: Add .tar.xz docs distributions

[StanFromIreland]

Also included updating the size of the html .tar.bz2 distribution to the current size.

• Issue: Documentation archives should be available also in tar.xz format #64940

---

📚 Documentation preview 📚: https://cpython-previews--129302.org.readthedocs.build/

[AA-Turner]

I'm not sure that this is worth it (as demonstrated by the issue being open for 11 years with very little demand). If we did this I would want to retire some of the older formats, and perhaps use Zstandard instead of LZMA, to overall reduce disk space -- but that is a larger discussion.

A

[StanFromIreland]

It is better than the current .bz2 by around 20% on the html.

[encukou]

I'm also not sure it's worth it, but that's for the issue.
If it is, the download sizes in the table and the “Unpacking” section below the table should also be updated.