docs: add v1->v2 migration guide to the index

[kandolfp]

In the index for the migration guides the new guide vor v2 was missing. I added the context as stated in the menu file _toc.json

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• @abbycross
• @beckykd

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[abbycross]

Thank you @kandolfp for catching this! It's highlighted a gap in how we track our files, so we are discussing the next step. If we close this PR, it's to address the problem in another one.