🤖 Store execution outputs in `Outputs`

[agoose77]

This PR closes #1674 by changing our internal representation of cell outputs so that they are children of the Outputs node.

[!Note] Part of initiative #1026

Notable Changes

• Address individual output children with labels ${block.identifier}-outputs-${N}
• Add new parent outputs node with identifier ${block.identifier}-outputs

Tasks

• [x] Check that .docx exports still export matplotlib figures
• [x] Check that outputs can still be embedded
• [x] Check that individual output nodes can be embedded
• [x] Check that individual outputs are still minified
• [x] Check that JATS exports still export matplotlib figures