User-guide: add doc to briefly describe Remediation feature in CAPM3

[fmuyassarov]

Add a document briefly explaining the Remediation feature in CAPM3.
Describe:

• briefly Remediation feature in CAPM3
• why we need it
• how to use it
• what are the caveats if any

/kind documentation
/help

[macaptain]

I spent some time learning about this not so long ago, so I can try and write some documentation on this feature.

/assign

[Rozzii]

/triage accepted

[macaptain]

Unfortunately, I won't have time to finish this.

/unassign

Relevant background for this documentation:

• Original proposal: https://github.com/metal3-io/metal3-docs/blob/main/design/cluster-api-provider-metal3/capm3-remediation-controller-proposal.md
• More interesting background details: Reboot remediation is incomplete cluster-api-provider-metal3#392
• Ansible feature test showing the fields used for configuration of the feature and the expected resources that get created by the controller:
  • https://github.com/metal3-io/metal3-dev-env/blob/master/vm-setup/roles/v1aX_integration_test/templates/machine-healthcheck-worker.yaml
  • https://github.com/metal3-io/metal3-dev-env/blob/master/vm-setup/roles/v1aX_integration_test/tasks/healthcheck.yml

[smoshiur1237]

/assign

[Rozzii]

Thanks @smoshiur1237
/remove-help