Document Metal3Machine and Metal3MachineTemplate

[vidarno]

Adds Custom Resources as a heading in the documentation and pages for Metal3Cluster, Metal3Machine and Metal3MachineTemplate.

Fixes #397

Note that I found #559 after I asked to be assigned this issue. It is fairly recent, so unsure if I should cancel this PR and if we should go with the other PR instead.

[metal3-io-bot]

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[metal3-io-bot]

Hi @vidarno. Thanks for your PR.

I'm waiting for a metal3-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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