[DOC] Create better info on staking pool contract for validator nodes

[janewang]

Is your feature request related to a problem? Please describe.
Validator nodes can make changes to the staking smart contract instead of all using the same ones. Certain amount of innovation can be done on this smart contract to attract stakes

Describe the solution you'd like
Better documentation on this https://github.com/near/core-contracts/tree/master/staking-pool to foster innovation

[zavodil]

Yep, this could be a game changer. Unclear whitelisting process for custom staking pool contract is the blocker here (from my perspective).