Implement support for beat.core description/documentation API

The new beat.core contains direct support for object descriptions and a documentation API.

Following the other functionality that is already well-merged with beat.core, it would be good that this package also used the beat.core APIs to set and get object descriptions and documentation.

The object short_description in beat.web (known as description in beat.core), can still be cached inside beat.web, but should be updated on beat.core once it changes. This allows users to simply edit the object JSON files and still be able to push the description to the server, when they are using the command-line interface.

Similarly, description in beat.web (known as documentation in beat.core) should be materialized in a proper file, like it is done for the JSON declarations. This would allow users from the command-line to edit this file and push it to the server.