Adding implicits and diagrams to scaladoc. See #2880

* This generates diagrams if the `dot` command from Graphviz is installed, but there is no way to fail the build if it doesn't find `dot`.
* We now check the generated HTML for diagram tags, and fail the build if we don't find any.

:exclamation: **You need to have Graphviz installed if you intend to run `package` or `unidoc`, or they will return an error**