Expand markdown editor page

[rrih]

Summary

• Expand the Markdown Editor page with paste-ready Markdown examples for README, release notes, and meeting notes workflows.
• Add a pre-download review checklist and FAQ entries for README usage and preview behavior.
• Keep ad density unchanged with the existing toolContent placement only.

Selected action

• Revenue Accelerator auto-pr: expand existing tool page https://rrih.github.io/tools/markdown-editor/.
• Control issue: Refs Revenue accelerator control tower #15.

Expected KPI impact

• Improve organic landing-page usefulness and query match for the Markdown Editor page.
• Target latest accelerator signal: 15 impressions and 0 clicks for the markdown editor URL.

Data rationale

• data/growth/accelerator/latest.md ranked the markdown editor expansion as the top available auto-pr existing-page action after draft-pr items.
• Open growth/revenue PR count before this PR: 2 draft PRs (Improve animation generator CTR copy #26, Add overtime pay calculator #28), both clean with successful CI.

Verification

• bun test - 27 tests pass
• bun --bun tsc --noEmit - pass
• bun run lint - pass with existing 9 warnings
• bun run build - pass
• bun run dev-check - pass, dev server ready
• Browser check at http://localhost:3333/tools/markdown-editor/ - desktop/mobile content present, no horizontal overflow, no console errors