* Add more margin above headers to visually associate them with the
prose below, rather than above.
* Reduce the margin around normal block elements to more clearly
associate them with one another.
* Increase the size of prose and medium/large headers to make them
easier to see.
* Reduce the font weight of intro paragraphs to make them more
visually distinct from the rest of the documentation.
Based on @slimekat's suggestions.
Closes#313