[WIP] 📖 docs: machinepool contract spec

[richardcase]

What this PR does / why we need it:

This adds documentation that details the contract for providers when implementing an infrastructure machine pool.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #12799

/area machinepool

TODO:

• Add and fix links

[k8s-ci-robot]

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[k8s-ci-robot]

@richardcase: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-verify-main	62cffb3	link	true	/test pull-cluster-api-verify-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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