Post-migration: Cloud security cleanup (#588)

Cleans up the Cloud Security section of the Elastic Security docs, fixes
syntax and image issues, removes unnecessary raw files and duplicate
pages, and reconciles serverless and ESS versions of pages. Also removes
D4C docs as per
[6198](elastic/security-docs#6198).

Affects all pages in the Cloud Security section.

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Co-authored-by: natasha-moore-elastic <[email protected]>
Co-authored-by: Nastasha Solomon <[email protected]>

Author: 3 people