[PP-1] Define a JSON API link relation for entities

Problem/Motivation

As noted in #2829746: Clean up \Drupal\jsonapi\LinkManager\LinkManager(Interface) → we want to simplify the link generation by creating a JSON API specific entity link.

Proposed resolution

As expressed in the linked issue:

I think \Drupal\jsonapi\LinkManager\LinkManager::getEntityLink() should not be necessary; instead the JSON API module should register a new link relation for every entity type. So that you can do $entity->toUrl('jsonapi').

Postponed by ✨ Make it possible to link to an entity by UUID Active