docs(site): reference ca2a llms.txt in the machine-readable summaries (#8)

ca2a.agentrust-io.com now serves /llms.txt (via mkdocs-llmstxt), so list it
alongside trace/manifest/cmcp for answer engines.

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

Author: imran-siddique

llms.txt

@@ -22,6 +22,7 @@ AgenTrust is organized around four complementary open standards. Agent Manifest
 - [TRACE llms.txt](https://trace.agentrust-io.com/llms.txt)
 - [Agent Manifest llms.txt](https://manifest.agentrust-io.com/llms.txt)
 - [cMCP llms.txt](https://cmcp.agentrust-io.com/llms.txt)
+- [cA2A llms.txt](https://ca2a.agentrust-io.com/llms.txt)
 
 ## Resources