Document contract for machine pools

[AndiDog]

Related to #12794.

We have documentation around machine pools such as the feature gate page and this page which somewhat describes parts of the contract and parts of the expected controller reconciliation workflow. For a new implementor though, it's hard to find out how to get started. The complete contract including optional parts (e.g. MachinePool Machines) should be revamped, and probably pages consolidated.

[sbueringer]

Makes sense. Pleas align to the format that we have for all the other contracts, e.g.: https://cluster-api.sigs.k8s.io/developer/providers/contracts/infra-machine

/area machinepool

/cc @mboersma @richardcase

[sbueringer]

/triage accepted
/kind documentation

[sbueringer]

/priority backlog

(feel free to re-prioritize)

[richardcase]

I can look at this

/assign