Revamp things-advanced (WSGI/ASGI) examples

We ought to showcase media handling in these instead of manually wrangling (de-)serialization. It is fine to show what is possible, but the emphasis should still be on media.

Revise App vs API usage: Useful for debugging problems in your API; :arrow_right: Useful for debugging problems in your App;.

Also check what's going on with the missing Content-Length in the ASGI variant, it looks like the proposed middleware doesn't handle it particularly well.