[Notebooks] Update docs with proper instructions for private notebooks updates

[kangp3]

What does this PR do? What is the motivation?

This PR updates our documentation for permissions management in Notebooks to align with the new experience introduced by the Private Notebooks initiative. We have a new modal and some new buttons to streamline permissions management, so these instructions have been updated to match this.

Merge instructions

Merge readiness:

• Org config default to true: https://github.com/DataDog/dd-source/pull/304407
• Downstream org config default true: https://github.com/DataDog/dd-source/pull/305697
• Enable feature flag for view-restriction checks across all orgs: https://mosaic.us1.ddbuild.io/feature-flags/notebooks-index-view-access-principals
• Enable feature flag for new permissions UX across all orgs: https://mosaic.us1.ddbuild.io/feature-flags/notebooks-permissions-ui
• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/kangp3/private_notebooks_permission_docs/notebooks/