Duplication / group issues of channel options and other concepts in the channels docs

[mattheworiordan]

We introduce the concept of channel params quite early in the channel docs, see https://ably.com/docs/realtime/channels#setting-channel-options. I would say this is too early given we want to help people understand the core concepts like publishing & subscribing first, but nonetheless we do that. That is covered in section 2.1 i.e. the very first thing a person learns about once they have seen the basic example. Then we talk about setting channel options in 2.9 (after we've covered 8 sections) see https://ably.com/docs/realtime/channels#modifying-options, yet we make no reference to what those options are in 2.1. This doesn't make sense IMHO.

We see a similar lack of continuity with channel lifecycle in 2.4 https://ably.com/docs/realtime/channels#channel-lifecycle, which doesn't actually cover the states of a channel or mention it. We then move onto 2.5 which is metadata, and then come back to attach related topics in 2.6 https://ably.com/docs/realtime/channels#implicit-attach with implicit attach. Then later we talk about channel states in 2.11 https://ably.com/docs/realtime/channels#channel-states, which is not linked up with the two prior topics that are related. In addition, none of the channel state stuff is grouped into say 2.X.Y where Y is everything to do with channel states.

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