Port the docs to Umberto

[mlewand]

Not that long ago we migrated most of CKSource projects to Umberto, so that all docs can be viewed in a unified format at https://docs.ckeditor.com/

Accessibilty Checker should also be ported to that format.

Back in the day AC had a dedicated page where instructions were in a separate articles. Currently after we launched new websites it was ported to one big article at https://docs.ckeditor.com/ckeditor4/latest/guide/dev_accessibility_checker.html. Content from there should be separated as a guides in AC project.

As for API docs it remains to be defined, as currently AC works only with CKE4 but we'll have AC for CKE5 at some point.