[Blocked by #256] Updates Levels and Requirements pages

[devmoran]

• Replaces existing content with 'ladders' format on levels
• Adds slsa badges on levels
• Remove duplicate definitions on requirements

Requires #256 for links to work.

Screenshot of new Levels content

Screenshot of tidied up Requirements top section

[MarkLodato]

Update: I just submitted #275, which is a replacement for #256 (blocker for this PR). Next step is for me to fork off this PR into a new one that incorporates both the original text and my suggestions.

[MarkLodato]

Closed in favor of #502.