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

[mattheworiordan]

I am bubbling this up as high priority for a number of reasons:

1. This is going to be used in our new LiveSync MongoDB connector to route data from change stream queries to channels.
2. Interpolation format is also used in batching, but not for routingKey but for a conflation key. So we have a conceptual model that needs explaining for our interpolation format. I assume we'd then explain on the interpolation format page where it's usde.
3. Do a search in Slack for interpolation and in support tickets and you will see customers are asking about this a lot. The fact that this is not discoverable is a problem that has not changed and is more relevant now than ever.