Use docsbuild-scripts

[m-aciek]

For now the workflow is assuming tags from CPython repo, but we should build from heads of version branches, to build with fresh changes (I think that's the docsbuild-scripts behavior as well?).

It would be easier to reuse the code of docsbuild-scripts to figure out list of Python versions and languages to build.

[m-aciek]

Hm, the docsbuild-scripts doesn't support PDF, and build other offline artifacts in one chunk, sequencially, I think, probably it isn't worth to use docsbuild-scripts, rather duplicate some logic from there 🤔