Fix non-docs pages accessible on docs subdomain

[Vincent-Ngobeh]

Summary

• Redirect /foundation/ and /community/ paths on docs.djangoproject.com to www.djangoproject.com
• Uses 301 permanent redirects to preserve SEO
• Preserves the full path and query string in redirects

Problem

Non-documentation pages (like /foundation/ and /community/) were incorrectly accessible on docs.djangoproject.com when they should only exist on www.djangoproject.com.

Fixes #878

Test plan

• [x] Added tests for /foundation/ redirect
• [x] Added tests for /community/ redirect
• [x] Added test for query string preservation
• [x] All existing tests pass