gh-139588: Docs: number the layers in execution model diagram

[m-aciek]

Prevents LaTeX PDF build error Too deeply nested. (Regression from #139509.)

• HTML build: https://cpython-previews--140445.org.readthedocs.build/en/140445/reference/executionmodel.html#general-computing-model
• PDF build screenshot:

• Issue: make dist-pdf is broken #139588

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📚 Documentation preview 📚: https://cpython-previews--140445.org.readthedocs.build/

[adorilson]

A list isn't a good representation of a layer architecture. Here, it will be wonderful if we have an image rather than a list (or something else.)

[m-aciek]

Closing in favour of #140709.