Backend > Vocabularies needs content

### Plone Documentation versions affected

- [ ] 5.x
- [X] 6.x

### Description

Backend > Vocabularies needs content.

Source material:

- https://github.com/plone/training/blob/main/docs/mastering-plone/registry.md
- https://github.com/plone/plone.app.dexterity/blob/master/docs/advanced/vocabularies.rst
- https://github.com/plone/plone.app.vocabularies?tab=readme-ov-file#overview