gh-154388: Document Python install variants

[BHUVANSH855]

Summary

This PR documents the CPython runtime variants available through py install.

It adds a short description of the published PythonCore, PythonTest, and PythonEmbed variants to the Installing runtimes section of the Windows documentation, clarifying that these are CPython-published runtime variants rather than features of the Python install manager.

Issue

Closes gh-154388.

Testing

• Built the documentation successfully with:

  make -C Doc html

• Verified that the documentation builds successfully with --fail-on-warning and that the new content renders correctly in the generated HTML.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33713657 | 📁 Comparing fe91943 against main (79cdcee)

  🔍 Preview build  

2 files changed

± library/warnings.html
± using/windows.html

[zooba]

That's a bit too literal a translation of the issue report. If you could, can you expand it to read more like instructive documentation for a new user - explain a bit more about each package, the circumstances when they may prefer one over the other, make a definitive recommendation (of the default), and show at least one example of the install and list --online commands for them?

[zooba]

Please don't force push. We always squash merge and rewrite commit messages at the end, so the history won't be in the repository.

All that force pushing achieves is to break incremental reviews - we now have to re-read your entire contribution and guess what has changed, rather than being able to see the difference from recent commits. This will slow down our review.