provide definitions for various WDL concepts

[a-frantz]

Tangentially related to #173

I'm reworking our preamble checks (in #187 ) and have found myself writing a very verbose explanation() for the PreambleFormatting rule. It's so long because it contains definitions for both lint directives and preamble comments, which are concepts that need to be understood to make sense of our preamble formatting. But these definitions shouldn't "live" within this explanation() text long term. There has to be a better place to put them and then we can link to them. However I'm a little stumped as to where that is.

This would be easier to solve if we had a website set up. Perhaps. Maybe not, because we want the website to be built from the Rust source, so that might not solve much.