Redesign "Constants" section

[jonathanhefner]

This changes the appearance of the "Constants" section to match the "Methods" sections.

Before	After

[p8]

This looks nice!
Code indentation seems to be ignored:

https://deploy-preview-356--sdoc.netlify.app/classes/activerecord

[jonathanhefner]

Code indentation seems to be ignored:

Unfortunately, that is value that RDoc::Constant#value returns. I believe it is due to the way parse_constant_body is implemented (specifically, due to the get_tkread_clean(/^[ \t]+/, '') calls). I'm not sure whether the end result is intentional or accidental.