Generalize index → source, make PyPI non-normative

[mgorny]

As an alternative to #94:

• replace "index" with "source", as I recalled Paul noted in the PEP 825 reviews
• rather than requiring one specific source, say "sources that are used", to cover @konstin point about using multiple indexes when installing some package
• replace the PyPI recommendation with a non-normative example

[read-the-docs-community]

Documentation build overview

📚 wheelnext-peps | 🛠️ Build #34804193 | 📁 Comparing 1d17026 against latest (344e589)

  🔍 Preview build  

51 files changed · + 6 added · ± 45 modified

+ Added

• pep-0845/index.html
• pep-0846/index.html
• pep-0847/index.html
• pep-9999/index.html
• pep-0825/appendix-metadata-consistency/index.html
• pep-9999/appendix-variant-json-schema/index.html

± Modified

• index.html
• numerical/index.html
• pep-0000/index.html
• pep-0426/index.html
• pep-0501/index.html
• pep-0534/index.html
• pep-0545/index.html
• pep-0585/index.html
• pep-0604/index.html
• pep-0654/index.html
• and 35 more...

[rgommers]

I like this, it's a further improvement over gh-94, which was an improvement already over the PEP text.