formalize convention for organizing translations.yml

[evanwill]

• how items in translations.yml match with the template file structure
• add comments in the yml with description to help users
• consider handling of repeating labels used in several locations (repeat configuration or reuse single key?)

[maehr]

I tried to match the folder and file structure in _data/translations.yml and decided not to reuse labels except on the same page. Regarding comments I have not found a good solution yet.