Add documentation link for ClassExtendsInternalClassRule

[mglaman]

We recently added ClassExtendsInternalClassRule which detects when code extends @internal (#248) released in 1.1.2.

Turns out that Drupal's @internal policy can be far-reaching, such as forms.

Here is the policy on Drupal core and @internal: https://www.drupal.org/about/core/policies/core-change-policies/drupal-8-and-9-backwards-compatibility-and-internal-api#internal

Core modules contain many route controllers that are bound to individual routes, as well as form classes. These controllers are not part of the API of the module unless specifically marked with an @api tag on the method or class.

I think this project needs to provide some kind of documentation on what to do when this rule is activated and what it means, providing more context than the policy itself.

[mglaman]

The most common occurrence is due to \Drupal\Core\Entity\ContentEntityDeleteForm which extends ContentEntityConfirmFormBase but the code generation tool used by Console and Drush extend ContentEntityDeleteForm (I think Commerce always does as well.)

This class has been marked @internal and is to be discussed in https://www.drupal.org/node/2491057

[mglaman]

Related issue in Open Social: goalgorilla/open_social#2661

[mglaman]

We should only link to the Drupal core policy if the source class is from Drupal core.