README: update how to build the documentation locally

[dilyanpalauzov]

Calling just

npm run serve-preview

prints

  ➡️  docs/installation/openhabian-backup.md
  ➡️  docs/installation/openhabian-exim.md

> openhab-docs@4.2.0 build-only
> vuepress build .

sh: line 1: vuepress: command not found

[netlify]

✅ Thanks for your pull request to the openHAB documentation! The result can be previewed at the URL below (this comment and the preview will be updated if you add more commits).

Built without sensitive environment variables

Name	Link
🔨 Latest commit	1bb8986
🔍 Latest deploy log	https://app.netlify.com/projects/openhab-docs-preview/deploys/68e8e33335423f0008cdd159
😎 Deploy Preview	https://deploy-preview-2584--openhab-docs-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[stefan-hoehn]

@pgfeller WDYT?

[pgfeller]

@pgfeller WDYT?

LGTM 👍