Update Pepr docs

[schaeferka]

Describe what should be investigated or refactored

With the root level markdown files now being pulled directly into the Pepr docs website, we need to review/update the markdown files in the Pepr Core docs folder to remove duplicate content, update links, and revise as needed for consistent style.

Links to any relevant code

(optional) i.e. - https://github.com/defenseunicorns/pepr/blob/24f1f253b1db4c9d97717c0282b8f39c74d3ace1/README.md?plain=1#L3

Additional context

Add any other context or screenshots about the technical debt here.