Tidy up docs structure

[sanity]

Near‑term structural tidy‑up

• Remove duplication in dev / developer-guide

  • Move everything under resources/manual/developer-guide into /dev (or vice‑versa).
  • Delete the duplicate section after the move.

• Adopt Diátaxis split

  • Under resources/manual create four top‑level buckets:
    get-started/, concepts/, how-to/, reference/.
  • Re‑file existing pages into the right bucket.

• Replace repeated blurbs with links

  • On the Platform page, trim the contract explanation to a short teaser and link to concepts/contracts.
  • Audit other pages for duplicated text and replace with a link to the canonical doc.

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Longer‑term polish

• Set up component‑level reuse so shared snippets (install commands, etc.) come from a single Markdown partial.
• Add audience filters (user, operator, developer) so readers can toggle views in the sidebar.