docs: state that adopting service discoverability opts an app in to ICP MCP

The protocol's whole purpose is that an agent handed only an app's URL can
work the rest out unattended, so publishing the manifest is the moment an
app becomes usable by agents that read it. Say so on the page, above the
fold, rather than leaving it implicit: adopting the protocol means the app
agrees to be discovered by ICP MCP under its terms of service.

Placed as a caution aside right after the intro, before the layer
breakdown, so it is read before any of the instructions it qualifies.

Author: claude

docs/guides/frontends/service-discoverability.md

@@ -9,6 +9,10 @@ When an AI agent is handed only your app's URL (for example, `https://yourapp.co
 
 This guide describes what a canister app exposes to make that possible, ordered by priority.
 
+:::caution[Adopting this protocol opts your app in]
+By implementing this service discoverability protocol for your app, you agree that your app should be discovered by [ICP MCP](https://internetcomputer.org/icp-mcp/), and that you respect its [terms of service](https://internetcomputer.org/icp-mcp/terms/).
+:::
+
 ## The five layers
 
 An agent handed only your app's URL should be able to do five things, unattended: