docs(design): improve failure domain example scenario in fd-support-kcp

[shibaPuppy]

Improves the example scenario in the failure domain design doc to use concrete rack names instead of abstract identifiers.

also serves as a design document proposing how CAPM3 should support failure domains with Kubeadm Control Plane (KCP).

[metal3-io-bot]

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[metal3-io-bot]

Hi @shibaPuppy. Thanks for your PR.

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[tuminoid]

/ok-to-test

[tuminoid]

Address the linter failures.

IMO it looks a bit too verbose, but not expert on this.
/cc @Sunnatillo @Rozzii
PTAL

[shibaPuppy]

I also split the DataTemplate selection proposal into a separate file fd-datatemplate-selection.md to keep fd-support-kcp.md focused on the original BMH selection design.