Data Entity: Guidance Improvements for Screenreaders

## Summary

- [ ] Improve guidance on our website in the [Interaction Behaviour](https://design.sis.gov.uk/components/data-entity#interaction-behaviour) for Data Entities regarding the use of an Edit button.
- [ ] Update code examples to reflect this new guidance, moving focus when the "edit" buttons are clicked.

(this ticket should have been raised against the Design System, sorry!)

## 💬 Description
New guidance should recommend an Edit button moving focus to the current data entity (if button is slotted in entity) or to the first data entity (if button is at the end of the group)

Improve code examples for Data Entities to reflect this guidance and have focus moved by the edit button to the freshly editable entity.

Start with the linked Data Entity Interaction Behaviour section and locate the website guidance and code examples it references. Update the Edit-button guidance and examples for the two described button positions, with completion shown by the documented focus behavior matching the examples.

Written by the indexing model from the issue text.