Old Ably diagrams are confusing

[mattheworiordan]

See https://ably.com/docs/core-features/pubsub and https://ably.com/docs/core-features/channels#understanding-decoupled-clients, both of them use this diagram:

I honestly have no idea what this diagram is trying to show users.

Given this is our Concepts documentation, so arguably the first stop for users who need to understand what a channel / pub/sub is, I believe we're setting new users up for confusion as opposed to clarity.

Looking through the docs more widely, I think a lot of the diagrams are dated and ineffective (using old styles, but more importantly, conceptually not strong). We have an opportunity to really show people quickly how Ably works in our concept docs, and arguably to some degree even in our SDK docs in the intro sections. We should strive to use richer media (GIFs, interactivity, clear diagrams etc.)

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