feat: render orientation content dynamically from community repo

[TineoC]

This PR configures the site to fetch the New Contributor Orientation content dynamically from the kubernetes/community repository.

Changes:

• Removed static content/en/docs/orientation/_index.md
• Added .gitignore to content/en/docs/orientation/ to manage fetched content
• Updated external-sources/kubernetes/community mapping
• Updated Makefile to include orientation directory in clean-all target

Depends on: kubernetes/community#8809
Closes #631

[k8s-ci-robot]

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