fix(docs): make mike versioned URLs slash-safe

[MSKazemi]

Summary

Fix MSKazemi/brain#29 from current main.

• normalize mike's slashless versioned site_url before appending paths
• fix the llms.txt alternate, Twitter preview image, changelog banner, and breadcrumb ancestor URLs
• compare the homepage canonical against the normalized base so a no-slash site_url cannot create a duplicate homepage breadcrumb
• exclude docs/overrides/** from published documentation so raw Jinja is not served
• make the SEO checker base-URL aware
• audit the three affected references site-wide
• add a CI build that reproduces mike's no-trailing-slash site_url and validates the rendered output

This preserves the current benchmark/scoring behavior and only changes documentation generation/validation.

Substantial AI assistance was used to prepare and verify this patch.