docs: add importable example workflows (levels 1-4)

[OndrejDrapalik]

Summary

• Add four importable n8n example workflows under docs/examples/, ordered simplest → most advanced:
  • Level 1 — run a command in a one-shot ephemeral sandbox
  • Level 2 — write + start a web server, return a preview URL
  • Level 3 — clone a repo, build, download the artifact back into n8n
  • Level 4 — AI Agent controlling one persistent sandbox via four E2B tools; the sandbox is found across chat runs with the new Get Sandbox → By Metadata server-side filter (feat: get sandbox by metadata filter #6) and paused between runs
• Add docs/examples/README.md with import instructions + per-example descriptions, and link the set from the main README

Test plan

• Import each JSON via Workflows → Import from File, attach an E2B API credential
• Levels 1–3: run via Test workflow, verify green end-to-end
• Level 4: attach an OpenAI credential, send the example chat prompt twice — run counter should increment (sandbox persists via metadata lookup + pause)