feat: (structure redesign) platform docs restructuring for `4.0`

[Piotr1215]

Summary

This PR reorganizes the remaining sections of the platform docs improving navigation and structure.

• Administer
• Understand
• Learn How-To
• Use Platform

Key Changes

• moving, renaming and restructuring
• no content updates, just small typos and naming
• adjust vale style rules
• fix vale style violations

Dependencies

• PR #269 must be merged first to avoid conflicts.

TODO

• [x] re-flowing the menu order
• [ ] convert @site calls to relative paths

Closes DOC-253

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[Piotr1215]

@deniseschannon thank you for the commit, I managed to fix all broken links and references in the last commit. Re-tagging you for review.

[deniseschannon]

Closing in favor of #279