Update Pipelines documentation

[ptitzler]

This PR updates the overview and pipelines user guide:

• Updates the existing content for version 1.2 (notebook pipelines => AI pipelines (notebooks + Python scripts)
• Removes some duplicate content and links to more details in other parts of the documentation. For example, the [pipelines] overview now links to the user guide and the tutorials and no longer discusses node properties.
• Adds a new pipeline export section
• Closes Document pipeline export and how to consume an exported pipeline #926

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