Update distutils docs references

[wayyoungboy]

Summary

• remove the Python 3.11 intersphinx workaround added for missing distutils objects in current CPython docs
• update the remaining distutils configuration-file reference in easy_install docs to point at Setuptools' self-hosted distutils docs

Fixes #4081.

Validation

• gh issue view 4081 --repo pypa/setuptools --json number,title,state,assignees,url confirmed the issue is open and unassigned
• gh pr list --repo pypa/setuptools --state open --search '4081 OR distutils CPython docs OR "Remove references to distutils"' --json number,title,author,url,headRefName returned no open related PRs
• git diff --check -- docs/conf.py docs/deprecated/easy_install.rst
• /opt/miniconda3/bin/python3.13 -m py_compile docs/conf.py
• focused Python assertion that docs/deprecated/easy_install.rst links to distutils/configfile and that docs/deprecated/distutils/configfile.rst exists
• rg check for the removed CPython distutils-doc workaround and old docs.python.org/install link returned no matches

Full tox -e docs was intentionally not completed because its dependency installation became too heavy for the shared worker machine; the long-running docs/tox pip process was stopped and the partial .tox/docs environment was removed.