docs: fix tox package_env references

[raisulchowdhury]

Summary

• Replace both unresolved local :ref:package_env`` references in the changelog with the existing external Tox intersphinx role.
• Keep the duplicate generated changelog entries consistent.
• Add the required changelog/14818.doc.rst fragment.

Closes #14818

Validation

• git diff --check
• git show --check HEAD
• Confirmed doc/en/conf.py already defines the tox intersphinx mapping.
• The external Chronographer changelog check initially required a news fragment; changelog/14818.doc.rst was added and pushed in c1dd525.
• The full docs build was not run because the host ran out of temporary disk space while provisioning tox; no temporary build artifacts remain.

Contribution policy

This is a small documentation fix. I personally reviewed the diff and existing Sphinx configuration, understand why the external role is required, and will respond to review feedback. AI assistance was used and is credited by the Co-authored-by: OpenAI Codex <noreply@openai.com> commit trailer.

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