Document line-spacing/justification rules

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And organize code to make them easily modifyable by other people.

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Initial documentation here, and I will be moving the "tolerance" parameters out of Javascript and into more human-accessible Jekyll data files.

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Moved the line-height and justification rules out into a new file _data/calculations.json. For now it is still JSON since that is obviously easier to load into the live Javascript. If this proves too unreadable for people to need to be able to read it, I can try to come up with a better format.