chore: update _redirects to match new sitemap (27 vs 28) and reorganized docs (#1755)

- Added redirects for old capabilities pages (hosted-authenticate-service, mtls-clients, etc.).
- Updated Helm references to point to /docs/deploy/k8s/install.
- Removed outdated references (pre 0.20) and consolidated rules to avoid 404s.
- Grouped rules logically by doc category for easier maintenance.

Follow-up to [#1726](#1726)

Author: desimone