Mention the unsupported web APIs on the Deno Docs landing page

Currently, anyone interested in Deno Deploy who reads the [marketing page](https://deno.com/deploy) or the first few pages of the [Deploy Docs](https://deno.com/deploy/docs) might assume that anything that works in Deno will work in Deno Deploy. That is not the case, and it only becomes obvious when you run into a deployment failure or you scan the full docs until you find this runtime [API Reference](https://deno.com/deploy/docs/runtime-api) page:

> It's different from Deno but aims to have similar APIs where applicable.

My suggestion is to make that fact stand out when entering the documentation, instead of leaving those important details buried. I found at least 2 [feedback ](https://github.com/denoland/deploy_feedback/issues/148#issuecomment-1020877437) [issues](https://github.com/denoland/deploy_feedback/issues/171) that were submitted because people weren't aware of the unsupported APIs.

For context, I ran into this because my toy project uses the Web Worker API. It works locally with Deno CLI, but Deno Deploy crashed with `ReferenceError: Worker is not defined`.