CM Document: do not show options for 'entity_documented' for anything that already has a document.

Problem/Motivation

The select list for "Documentation for (entity_documented) lists all documentable entities, even the ones that are already covered by a cm_document. Not only is this a recipe for a problem because there should not be duplicate entities, it creates an annoying user experience. That list should get shorter as more things are documented.

Caveats:

1. The list should never remove the currently selected item from the list. (would make it impossible to edit something that has already been saved)
2. The list should never remove site.note, site.principle, and site.process as those can have duplicates.

This is a related, but slightly separate issue from the actual validation issue here.
https://www.drupal.org/project/content_model_documentation/issues/3339247

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes