Use yaml for package data instead of json

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This is a continuation of sorts of #567 - I noticed a lot of issues in the md-variables.json file probably arose due to how difficult it was to edit arrays of strings! YAML isn't everyone's favorite, but I think it really shines here with the multiline strings. This PR converts the md-variables.json file to YAML, including my typo fixes from #567, and updates the scripts/generate-readmes.js script accordingly to parse the YAML.