docs: collapsed example

[mortenpi]

Takes the demo code snippet from here and implements it as a local extension module that adds support for simple collapsed at-example blocks to docs.

@baggepinnen: I would suggest you take over the PR and actually apply it in the places where you want. And let's think about a package in the next iteration.

Note: this does bump Documenter to the current latest commit on master. But it should just bring some small UI changes, nothing major.