Implementers' Guide: Chain Selection (#3262)

* high-level discussion of fork-choice and chain selection

* notes on chain-selection subsystem

* send `Approve` messages from approval-voting

* remove references to candidate-selection

* adjust grandpa voting rule docs

* Update roadmap/implementers-guide/src/node/grandpa-voting-rule.md

Co-authored-by: Lldenaurois <ljdenaurois@gmail.com>

* Update roadmap/implementers-guide/src/protocol-chain-selection.md

Co-authored-by: Lldenaurois <ljdenaurois@gmail.com>

* Update roadmap/implementers-guide/src/protocol-chain-selection.md

Co-authored-by: Lldenaurois <ljdenaurois@gmail.com>

Co-authored-by: Lldenaurois <ljdenaurois@gmail.com>