Proposal for security overview page

[pipo02mix]

What this PR does / why we need it

Towards https://github.com/giantswarm/giantswarm/issues/30733

Things to check/remember before submitting

• If you made content changes

  • Run make dev to render and proofread content changes locally.
  • Bump last_review_date in the front matter header if you reviewed the entire page.

[stone-z]

We haven't started on this doc yet :sweat_smile: but we will take that into consideration