docs(site): add Confidential A2A (cA2A) to agentrust-io.com (#5)

Add cA2A alongside TRACE, Agent Manifest, and cMCP across the homepage:
product card, supernav, "At a Glance" table, JSON-LD standards ItemList, a
"What is cA2A?" FAQ entry, and the AgenTrust / relate / open-source answers.
Also updated meta descriptions and llms.txt.

cA2A is a developer preview and its docs subdomain is not live yet, so links
point to the GitHub repo (not ca2a.agentrust-io.com) and it carries a Preview
badge, not Live. Swap links to the subdomain once DNS is configured.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: imran-siddique