Fix security top-level overviews

[shainaraskas]

made the security overview page much shorter

pages to review:

• security overview
• self-managed cluster security overview
• encryption overview

and some small style polish things

todo:

• explain when to use self-managed config instructions
• clarify cipher suites (I suspect these are only applicable to self-managed and maybe eck, and then they just work out of the box for other deployment methods)
• secure settings generic/eck split
• the ECE security overview
• private link traffic filters
• clarify secure http communications

[eedugon]

Feels very good! I've just added some comments for the encryption at rest page, which still looks a bit confusing.

And in a separate thread I'll share feedback about the minimal, basic and basic + HTTPS concepts, because they were not part of this PR.

[eedugon]

LGTM!!!