Automatically generate HTML namedTypes/builders documentation from the declarations in def/*.js

We already use the `def` syntax to generate AST types and builders automatically, so it would be awesome if we also generated documentation at https://benjamn.github.io/ast-types/documentation, or something like that.

Start by reading the declarations in def/*.js and the existing AST type and builder generation described in the issue. Determine how documentation should be generated and published at the ast-types documentation URL. Done means the namedTypes/builders documentation is automatically produced from those declarations.

Written by the indexing model from the issue text.