Handle /page/ redirects for subprojects

[ericholscher]

Summary

• document the spike to restore /page/ redirects on subprojects
• make ServePageRedirect resolve subprojects from either the current project, alias, or slug
• add a regression test covering /projects/<subproject>/page/ redirects
• document how to run the subproject page-redirect test with the proxito settings
• fix a ruff lint warning in ServePageRedirect by removing an unused import

Testing

• ✅ DJANGO_SETTINGS_MODULE=readthedocs.settings.proxito.test uv run pytest readthedocs/proxito/tests/test_old_redirects.py::InternalRedirectTests::test_page_redirect_on_subproject -vv -s
• ✅ uv run ruff check readthedocs/proxito/views/serve.py readthedocs/proxito/tests/test_old_redirects.py

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Codex Task

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