Create `@example` "include" pre-processor feature for java/scala-doc

We want to use this to power examples for all directives. It will make it easier to maintain the directive docs and would also address https://github.com/akka/akka/issues/18660

The idea is to keep examples somewhere where they are compiled as usual. Then we'd put a reference to an example directly into the scaladoc and then have a preprocessor (on sbt or scaladoc/javadoc level) that includes the snippet at that point into the scaladocs.

Example would be:

class ThingDirectives {

/**
 * Does things.
 *
 * @example $root/akka-http-docs/blabla/ExampleDirectivesSpec.scala#example
 */
def example(t: String): Directive0 = ???
}

It should:

• Fail compilation if include does not exist
• Work for Scaladoc and Javadoc
• create a <code> block basically
• in Scaladoc there was an example feature I believe, check that
• allow us to move all directive method docs into scaladoc, so we don't have to maintain explicit docs pages for each of them