Sphinx :base-ref: references are broken by ABI hashes

The user guide's :base-ref: directive is used to generate hyperlinks to the Haddocks for declarations in base. It does so by deriving the expected relative paths to Haddock documentation in docs/users_guide/conf.py from knowledge of bindist structure, the package name, and version. Unfortunately, these paths are now wrong as package directories now include an ABI hash.

Originally reported in #23988 (closed).