Move the Realtime protocol documentation to this repository

[QuintinWillison]

It's currently:

• Defined as source code in our docs repository as protocol.textile
• Rendered for preview at docs.ably.com
• Rendered alternatively at ably.com/docs - albeit less discoverable there, due to apparent lack of exclusion in site menus (this is probably intentional)

It should be converted to markdown format and moved to this repository, so that it can be atomically managed here as a peer-level content member.

┆Issue is synchronized with this Jira Uncategorised by Unito

[QuintinWillison]

I'm currently on the fence as to whether this will be moved here. I may well need to produce an ADR around this, for discussion with @SimonWoolf, @owenpearson and @lmars, plus others.

[QuintinWillison]

Since my last comment on this issue, I authored and got the decision on ADR66 "New home for the features specification" (internal document). It was that decision that prompted the creation of this repository (ably/specification) as well as the movement of this issue to this repository.

[QuintinWillison]

The protocol documentation was moved here within the work undertaken for #1.
I've decided to keep it in textile format for now.

Therefore, there is no further work required on this particular issue, so closing.