Add canonical URL headers to resolve dual-deployment deduplication

[abrahamdaniels]

The dual Mintlify deployment causes agents and search engines crawling docs.mellea.ai to encounter duplicate content at old and new URLs. Without <link rel="canonical"> headers, crawlers treat these as separate pages, splitting link equity and increasing the chance of surfacing stale content.

Fix: Add canonical URL headers to all pages pointing to the new URL structure. In Mintlify, this is typically a canonicalUrl metadata field. Verify via curl -I https://docs.mellea.ai/overview/quick-start that the canonical header is present and points to the new equivalent URL.