docs(docs-next): port comparison + faq + changelog (phase 4a)

[pratyush618]

First slice of Phase 4. The original plan estimated Phase 4 at "1–2 days" but the source totals ~10,700 lines across 71 pages — way more than fits in one reviewable PR. Splitting into multiple slices.

This PR ports the three standalone pages that show in the navbar's "More" dropdown:

• `docs/comparison.md` → `docs-next/content/docs/more/comparison.mdx` (feature matrix + per-engine deep dives)
• `docs/faq.md` → `docs-next/content/docs/more/faq.mdx` (13 entries)
• `docs/changelog.md` + `docs/changelog/archive.md` → `docs-next/content/docs/more/changelog.mdx` (merged, with 0.1.0–0.5.0 collapsed into a `
  Details

  ` block per the approved IA)

Why these three first

• They're small (775 inserted lines vs ~10,700 to come) and reviewable in one sitting
• The "Changelog Archive" / "Changelog" merge was an explicit IA decision in the plan — landing it now removes the visible papercut
• Comparison + FAQ are high-traffic landing-from-Google pages
• Demonstrates the conversion pattern for the bigger slices that follow

Phase 4 plan (remaining slices)

• 4b — Guides / Core (7 pages)
• 4c — Guides / Reliability (7 pages)
• 4d — Guides / Advanced Execution + Operations (15 pages)
• 4e — Guides / Observability + Extensibility + Resources + Workflows (25 pages)
• 4f — Integrations (7 pages)
• 4g — Examples (7 pages)

Each will be its own focused PR.

Test plan

• `pnpm types:check` — clean
• `pnpm lint` — exit 0 (2 known scaffold-CSS warnings, no errors)
• `pnpm build` — clean, no new warnings
• `
  Details

  ` collapser renders correctly in the changelog (Fumadocs supports HTML5 `details` natively)
• CI: `Build Docs (Next)` job stays green
• Manual smoke: navigate to `/docs/more/` from the navbar dropdown — Comparison + FAQ + Changelog all resolve, archive expands cleanly