PEP 808: add discussion link

[henryiii]

This adds Discussions-To with a link to https://discuss.python.org/t/pep-808-partially-dynamic-project-metadata/104883, followup to #4598.

• 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)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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📚 Documentation preview 📚: https://pep-previews--4707.org.readthedocs.build/

[hugovk]

Thanks!