template: add Flatcar template

[tormath1]

What type of PR is this?

/kind feature

What this PR does / why we need it: In this PR, we add Flatcar templates and documentation for folks interested to use Cluster API with Flatcar without using pre-built images.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): #1325

Special notes for your reviewer: This has been tested with Kubernetes 1.30 (as 1.31 is failing currently due to the missing external CCM)

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add Flatcar template

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@tormath1: 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-provider-gcp-e2e-test	f803561	link	true	/test pull-cluster-api-provider-gcp-e2e-test

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