WebMCP documentation is out of date

### Existing documentation URL(s)

All code snippets in https://developers.cloudflare.com/browser-run/features/webmcp/ reference `navigator.modelContext`, however we're recently moved this API to `document.modelContext`: https://github.com/webmachinelearning/webmcp/pull/184.

### What changes are you suggesting?

While some experimental Chrome versions of the past still have the `navigator.modelContext` API, that is not what all new Chrome versions have, and is not what would ship, so it'd be great to update the docs to reference `document.modelContext()`!

Also note that `registerTool()` now returns a Promise, so it might be good to demonstrate `await`ing it in the docs.

### Additional information

_No response_