Guide: Candidate Backing Subsystem final cleanup

[rphmeier]

This is already fleshed out to some extent, but needs to be adapted to update its interactions with statement distribution, candidate selection

[rphmeier]

Note that this serves roughly the same purpose as the existing SharedTable, and can leverage the polkadot-statement-table crate - quorum-tracking logic doesn't need reimplementation.

This quorum tracking logic includes misbehavior detection of different kinds

[rphmeier]

The Candidate Backing System was the first to be written up, and unsurprisingly in the last few weeks' changes and improvements to the guide, it has become somewhat outdated w.r.t. the protocols and other subsystems that have evolved.

The remaining work to be done here:

• Change to match the Description/Protocol/Functionality/Jobs format of other subsystem pages
• Add links to message types
• Replace the section on SpawnValidationWork with asynchronous logic that combines the PoVDistribution and CandidateValidation subsystems (see Guide: PoV Distribution - merge with statement distribution or find clean interface for learning about peer knowledge #1228, this might be StatementDistribution instead).
• Note that interactions with Runtime API are with the Runtime API subsystem and detail these interactions. Add more API request types to Runtime API if needed.
• Note that this is the only subsystem that should be signing statements.