📖 Add security guidelines for Cluster API users

[nayuta-ai]

What this PR does / why we need it:
Add the security guidelines document for Cluster API users.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #6152

/area documentation

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[nayuta-ai]

/area documentation

[sivchari]

/cc @sbueringer @fabriziopandini

[fabriziopandini]

@nayuta-ai this is awesome!
It is a great starting point for most of the CAPI users, and we can always iterate and add more details if there is demand

[fabriziopandini]

What if we add a link to https://github.com/kubernetes/sig-security/blob/main/sig-security-assessments/cluster-api/self-assessment.md#threat-modeling-with-stride somewhere?

[fabriziopandini]

/ok-to-test