fix(marketing): redirect self-hosting guide to canonical docs

[backnotprop]

Summary

• redirect the legacy /docs/guides/self-hosting route directly to https://docs.plannotator.ai/open-source/self-hosting
• keep the existing 301 response, cache policy, and trailing-slash normalization
• add a focused test for the exact target and trailing-slash variant

The destination shipped in plannotator/docs PR #34 and now returns HTTP 200. This change does not modify the marketing hero, static pages, assets, metadata, or any other redirect.

Verification

• bun test apps/marketing/cloudfront/url-rewrite.test.ts: 6 pass, 0 fail
• node --check apps/marketing/cloudfront/url-rewrite.js: passed
• bun run build:marketing: passed, 40 pages built
• git diff --check: passed
• CloudFront Function source: 4,414 bytes, below the tested 10,000-byte limit
• https://docs.plannotator.ai/open-source/self-hosting: HTTP 200 with the expected self-hosting title and canonical URL

The currently deployed legacy route still returns the old sharing-anchor target. After this PR is reviewed, merged, and deployed, the CloudFront Function will return the new canonical Docs URL in the same single 301 hop.