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

[k8s-ci-robot]

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[tormath1]

/test pull-cluster-api-provider-gcp-e2e-test

[salasberryfin]

To make this new section available in the book, it needs to also be added to the list of topics in ./docs/book/src/SUMMARY.md here, maybe as a new sub-section under Self-managed clusters?

[tormath1]

Perfect. Thanks a lot, it makes more sense to have it here indeed.

[tormath1]

/test pull-cluster-api-provider-gcp-e2e-test

[k8s-ci-robot]

@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	9c83f61	link	true	/test pull-cluster-api-provider-gcp-e2e-test

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