= docs: add links from directive usages in examples to docs, fixes #703

Two components:
 * client-side javascript which adds the links
 * build-component that tries to extract the set of directives a source
   file defines

The extraction works by a heuristic regexp that tries to distinguish directive
definitions against other definitions. Directives can explicitly be marked
as such by prefixing them with a block comment, `/* directive */`.