Generate documentation redirects for lowercase rule codes

[InSyncWithFoo]

Summary

Resolves #15016.

Test Plan

None.

[charliermarsh]

Thanks.

[InSyncWithFoo]

I couldn't actually verify if this works locally (even before the change), but theoretically it should.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

warning: Detected debug build without --no-cache.
error: Failed to read examples/Assistants_API_overview_python.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: expected `,` or `]` at line 197 column 8

Formatter (preview)

ℹ️ ecosystem check encountered format errors. (no format changes; 1 project error)

openai/openai-cookbook (error)

ruff format --preview

warning: Detected debug build without --no-cache.
error: Failed to read examples/Assistants_API_overview_python.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: expected `,` or `]` at line 197 column 8

[dhruvmanila]

Thanks!

I tested it out locally using by generating the docs using:

uv run --with-requirements docs/requirements-insiders.txt scripts/generate_mkdocs.py

and, checking whether the mapping was created in mkdocs.generated.yml and running the server using:

uvx --with-requirements docs/requirements-insiders.txt -- mkdocs serve -f mkdocs.insiders.yml -o