Docs for ext MCP

[artberger]

Docs for the new 1.3 release feature, ExtMCP backend policy for external auth and external processing on MCP requests.

Related issues:

• Dev issue: Design a MCP-aware ext_authz agentgateway#175
• Initial implementation: feat(mcp): mcpGuardrails with ExtMCP for MCP aware extauth/extproc  agentgateway#1842

Example release note text:

Agentgateway now supports ExtMCP, allowing MCP-aware external auth and external processing integrations. This lets external services make decisions using MCP request context rather than only generic HTTP metadata.

Existing docs: for ext auth and ext proc, they are HTTP-based traffic guides, not MCP specific.

• kubernetes mode ext auth: https://agentgateway.dev/docs/kubernetes/main/security/extauth/basic/
• kubernetes mode ext proc: https://agentgateway.dev/docs/kubernetes/main/traffic-management/extproc/
• kubernetes mode MCP section: https://agentgateway.dev/docs/kubernetes/main/mcp/
• standalone mode MCP section: https://agentgateway.dev/docs/standalone/main/mcp/
• standalone ext auth: https://agentgateway.dev/docs/standalone/main/configuration/security/external-authz/
• standalone ext proc: https://agentgateway.dev/docs/standalone/main/configuration/traffic-management/extproc/