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
  • JSON API
  • Legacy API: docs: migrate index/upload API docs to user docs #17123
  • Stats API
  • XML-RPC (N/A?)
  • Integration guide (N/A?)
  • BigQuery Datasets
  • 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.