docs: add hardening guide

[alegrey91]

What type of PR is this?

/kind documentation

What this PR does / why we need it:
The PR provide a brief guide to easily harden the cluster with the latest options added on kubespray.

Which issue(s) this PR fixes:

Fixes #8808

Special notes for your reviewer:
Please, refer to the following discussion: #8773

Does this PR introduce a user-facing change?:

Add Hardening setup guide

[k8s-ci-robot]

Hi @alegrey91. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[oomichi]

No issue has been opened before.

We already have a corresponding issue #8808 😄

[alegrey91]

No issue has been opened before.

We already have a corresponding issue #8808 😄

Sorry, I totally forgot 😅

[oomichi]

Thank you so much for writing this document, that will be useful for users.
Most part seems good for me, just some comments.

/cc @oomichi

[oomichi]

@alegrey91
Thank you so much for your quick reply.
Please update this pull request based on the discussions.

[oomichi]

Thanks for updating.
Really nice work @alegrey91 👍

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alegrey91, oomichi

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[alegrey91]

Really nice work @alegrey91 👍

Thanks @oomichi :)
Seems that a job is in pending state from yesterday 😅

[cristicalin]

Love your recent work @alegrey91 !

/lgtm

[alegrey91]

Thanks a lot @cristicalin :)