Document routingKey interpolation somewhere more prominently

[jaley]

The only docs I can find for the interpolation string support in Firehose routing keys is this FAQ.

This is really hard to discover, and it's a real shame because this a great feature. I think it's actually a really important part of the learning journey for developers to picture how they can fan-out or fan-in as a function of their own data using Ably integrations. Personally, I think it should at least have a place on the main docs site and be linked from the various integration docs that support it, but it may be useful to include it in examples or other places where we're trying to get the value of integrations across.

┆Issue is synchronized with this Jira Task by Unito