[docs] Tidy up federation docs into 'federating with gotosocial' section

[tsmethurst]

Description

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This pull request tidies up some of our federation documentation, adds details of our sicko http signature stuff, and prepares the way for writing more detailed docs on what else remote instances need to know in order to federate nicely with GoToSocial.

Checklist

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• I/we have read the GoToSocial contribution guidelines.