feature: Editable redirects

[jbmoelker]

Changes

• Adds editable redirects to the CMS (Redirect Rule model).
• Adds postbuild script to download redirects and save to a _redirects file.
• Adds documentation on routing, including redirects.

Associated issue

Resolves #64 Docs part of #71

How to test

1. Review the new Redirect Rules model in the CMS
2. Create or edit Redirect Rules in the CMS (requires rebuild to take effect)
3. Open the preview deployment
4. Verify that the redirect rules work (also try reordering them):
   • /redirect-placeholder/:slug/ -> /en/overview-page/
   • /redirect-wildcard/* -> /en/overview-page/demos/tables/
   • /redirect-order/* -> /nl/
   • /redirect-order/static-slug/ -> /nl/gedeeltelijke-pagina-layouts/

Checklist

• [x] I have performed a self-review of my own code
• [x] I have made sure that my PR is easy to review (not too big, includes comments)
• [x] I have made updated relevant documentation files (in project README, docs/, etc)
• [x] I have added a decision log entry if the change affects the architecture or changes a significant technology
• [x] I have notified a reviewer

[cloudflare-pages[bot]]

Deploying with    Cloudflare Pages

Latest commit:	773150a
Status:	✅  Deploy successful!
Preview URL:	https://f47dd1ba.head-start.pages.dev
Branch Preview URL:	https://feat-editable-redirects.head-start.pages.dev

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