Document NHS button texts and Save and exit link

[alex-serednev]

Documents the NHS design system changes from HealthSamurai/sansara#8012 (PR sansara#8071).

aidbox-ui-builder-alpha/configuration.md

• Theme — added the new theme properties button.next-text and button.review-text.
• Design System — new NHS design system subsection: which button labels can be customized and how the Save and exit link behaves (saves without validation, returns to the NHS App home screen or redirects to redirect-on-save, and when it is hidden).
• Translations — added the form.button.review and form.button.redirect keys, plus a note that a label set in the theme wins over a translation.

aidbox-ui-builder-alpha/form-creation/form-customisation-in-theme-editor.md

• Buttons — noted that with the NHS design system only the button texts are editable, and listed the four fields shown in the editor.

Drive-by fixes

Three defaults in the translations example did not match the product anymore:

key	was	now
form.button.amend	Saved	Amend
form.button.next-item	Next	Continue
form.button.submit	Submit	Submit answers

Also, button.redirect-* was described as the "Close" button — the button is labelled Save in the renderer and in the theme editor, so the comments now say "Save" and mention that the same property is the Save and exit link in the NHS design system.

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