docs: add openHAB integration guide + Prometheus alternative

[LarsLaskowski]

Summary

Adds a concrete integration recipe for openHAB, which the README names as a
driving use case but previously shipped no wiring for.

• New docs/integrations/openhab.md with copy-paste HTTP-binding
  .things and .items that poll /api/v1/metrics once per refresh and
  extract CPU usage, load, CPU/GPU temperature, memory/swap, root-FS usage,
  available updates, uptime, and system identity via JSONPath. All field
  paths map 1:1 onto docs/API.md.
• Includes an API-key variant (X-Api-Key request header), a
  verify section (Thing goes ONLINE, live Items, example sitemap), and
  troubleshooting for the common OFFLINE / 401 / NULL cases.
• Adds the requested Prometheus alternative: since PiMonitor has no
  native /metrics endpoint, the guide uses
  prometheus-community/json_exporter in front of /api/v1/metrics, with a
  ready json_exporter.yml (including a per-mountpoint disk object) and the
  matching Prometheus scrape_configs relabeling.
• Links the new guide from README.md under a new Integrations heading.

Documentation only — no code, API, or config changes.

The example .things/.items were written against the documented
/api/v1/metrics response shape and JSONPath transformation semantics; they
have not been exercised against a live openHAB instance, so a follow-up
smoke test on real openHAB + Pi hardware is still worthwhile.

Related Issue

Closes #7

Checklist

• Tests added/updated for the change (go test ./... passes locally) —
  N/A, documentation-only; no Go changes (go build ./... clean)
• go vet ./... and golangci-lint run are clean — N/A, no code changes
• Documentation updated (this PR is the documentation)
• No breaking change to /api/v1/... response shapes — no API change