📖 Add documentation for MachineHealthChecks

[JoelSpeed]

What this PR does / why we need it:

Adds documentation for the recently implemented Machine Healthchecking

I've added a section on configuring a MachineHealthCheck for end users, plus a diagram in the controllers section.

Is there anything else you'd like added? Open to adding more if there are suggestions, just not sure what else would be useful

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #1990

[enxebre]

This is great, thanks!

[k8s-ci-robot]

@JoelSpeed: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-cluster-api-capd-e2e	011ed38	link	/test pull-cluster-api-capd-e2e

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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[rudoi]

Thanks, this looks great!

[vincepri]

/milestone v0.3.0

[vincepri]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JoelSpeed, vincepri

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