Addition of Onboarding page with logic of using dedicated Onboarding Project for onboarding tasks

[daniel-rdt]

Changes Proposed in This Pull Request

This PR adds information for onboarding to the onboarding page in the handbook. It is centred around the idea of using a dedicated Onboarding Project for each new team member in the open-energy-transition company GitHub. There you can find tasks that need to be completed by responsible OET members before the onboarding as well as tasks that the new team member will complete after starting.

An example project: https://github.com/orgs/open-energy-transition/projects/10 A project template based on this: https://github.com/orgs/open-energy-transition/projects/11

Checklist

• [x] I tested my contribution locally, and it seems to work fine.
• [x] Code and workflow changes are sufficiently documented.
• [x] If new pages are added, maintainers are assigned for that document.
• [x] If the document falls under a controlled category, a controlled document banner is present.

[netlify[bot]]

✅ Deploy Preview for handbookpr ready!

Name	Link
🔨 Latest commit	3d7561821354ce17ff2bc2c11de46113454040c3
🔍 Latest deploy log	https://app.netlify.com/sites/handbookpr/deploys/66daeb1b5398680008b80762
😎 Deploy Preview	https://deploy-preview-131--handbookpr.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 site configuration.