RFC Baking accounts proposal

This issue is for tracking Baking accounts feature proposal

• the feature is implemented in tezos!2458 (merged)
• the documentation is included in the Tezos docs in metastatedev/tezos!349 (merged)
• the feature has been running in Metastate's test network dubbed Astrolabe, the rendered docs that include metastatedev/tezos!349 (merged) can be found at https://metastatedev.gitlab.io/astrolabe-docs/