Move API documentation from Warehouse developer docs to PyPI user docs

[di]

Documentation for our APIs and datasets currently lives in our developer-facing docs: https://warehouse.pypa.io/api-reference/index.html

However, these are really end-user docs, and should live at https://docs.pypi.org/ instead.

[woodruffw]

Adding a roadmap:

• Index (Initial PEP 740 documentation #16398)
  • Feeds: docs: migrate RSS Feed docs to user docs #17171
  • JSON API: docs: migrate JSON API docs to user-docs #17178
    • Update links to the JSON API once migrated, e.g. in the integration guide docs: docs: update API links everywhere #17211
  • Legacy API: docs: migrate index/upload API docs to user docs #17123
  • Stats API: docs: move stats API to user docs #17161
  • XML-RPC (N/A?)
  • Integration guide: docs: move integration guide to user-docs #17173
  • BigQuery Datasets: docs: move BigQuery to user docs #17162
  • Integrity API (i.e. PEP 740, not currently documented): Initial PEP 740 documentation #16398

Once each of these is moved/added, it might make sense to add redirects from the old dev-docs URLs to the new user-docs ones.

[di]

XML-RPC (N/A?)

I think it would be wise to not bother porting these docs.

Integration guide (N/A?)

I think this can probably get included in the top-level page at https://docs.pypi.org/api/

[woodruffw]

All planned migrations are complete, so closing!