Add inline documentation to preview/admin

Any element with a data-documentation-part attribute becomes documentable. When option-clicked upon,
a dialog opens with the contents from a documentation provider configured in the documentation.yml config file.

There can be several types of documentation providers, currently the two types “static” and “json_based” are supported.

JsonBasedDocumentationProviderTypeModule is a work in progress but will delegate all documentation calls to an external JSON source.
StaticDocumentationProviderTypeModule has all documentation defined inline inside the config files.