PEP 12: Extract Intersphinx section. Add example of Intersphinx reference.

[davidfstr]

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
  • To an Active/Process PEP
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--3827.org.readthedocs.build/

[davidfstr]

Friendly ping to @brettcannon / @warsaw that this change to PEP 12 is ready for review.

• I split the former "Canonical Documentation and Intersphinx" section to two sections, "Intersphinx" and "Canonical Documentation".
• I extended the new "Intersphinx" section slightly, with an example of making an intersphinx reference.

[brettcannon]

I'm fine w/ this if @warsaw is.

[warsaw]

LGTM! Thanks.