Better entity documentation

[jhomarolo]

Is your feature request related to a problem? Please describe.
Today there is already a good documentation of entities, but it would be important to make some improvements.

1.- Entity link, that is, when an entity has property, a link from one to the other would be interesting.
2. - Highlight fields that have validation and explain what type of validation is happening there.
3. - A more explicit way of mapping which use cases use that entity as input or output.