📖 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

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Hi @nayuta-ai. Thanks for your PR.

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