extract message_type specs

[mixmix]

Writing the specs, I noticed that

• the message_type specs took up quite a lot of space
• I also noticed that it took some care not to leak concepts out of e.g. type 1 spec into the rest of the document
• I want to make type 2, type 3 which improve on these other types and explore the possibility space... and I don't want to spam or be fighting for attention/ space in the main document

So in the interests of keeping aspects of the spec "decoupled", and setting up a pattern for good growth, I propose we split out the message_type specs into their own documents in this repo.

[tschudin]

There is a beauty in SSB's "protocol guide" in the sense that it has everything, in one document. I hope we can have the same with tinySSB - it's the tiny version of SSB, after all. (I know, this is misleading, as going tiny made things more complex).

So what about working with an annex for the message type details, in the same document instead of splatter them?

[mixmix]

I personally think following one hyperlink is acceptable
I expect I'd personally like to deprecate some of the types in the future and say I support 3 + 5. We already see this in the implementations - type 0 is in the code but is just not used in some of the code bases, which makes it take up mind-space.

Happy to park (pause) this till it becomes a problem. It's very possible I'm prematurely optimising 😆