fix(docs): make docs.asdecided.com canonical

[tcballard]

What changed

• Set MkDocs site_url to https://docs.asdecided.com/ so generated canonical tags and sitemap URLs use the branded documentation host.
• Add a root robots.txt with the docs sitemap directive and explicit AI/search crawler access.
• Add a docs-specific llms.txt that identifies docs.asdecided.com as canonical and links to the authoritative documentation and source repositories.

Why

The docs host previously declared asdecided.github.io as canonical while GitHub Pages redirected that host back to docs.asdecided.com, creating conflicting canonical and sitemap signals.

Validation

• mkdocs build --strict passes locally after vendoring Core and specification sources.
• Generated sitemap uses https://docs.asdecided.com/ URLs.
• Generated canonical tags use https://docs.asdecided.com/.
• Generated robots.txt and llms.txt are present at the site root.