Fix divider width for small checkboxes

[colinrotherham]

Hello 👋

There might be some history behind this, but I noticed small checkboxes use the regular divider width (unlike radios)

This PR ensures checkboxes follow the same convention as radios and adds a new example

Before

After

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History

1. Dividers supported for Radios in GOV.UK Elements
2. Dividers added to Radios in GOV.UK Frontend via PR #849
3. Smaller Radios and Checkboxes added in PR #1125 with narrow divider widths in b645ac1
4. Dividers added to Checkboxes in GOV.UK Frontend via PR #2151 without narrow divider widths

Worth mentioning existing comments regarding divider widths:

• Radios govuk-design-system-backlog#59 (comment)
• Ability to group radio buttons / checkboxes within sub-headings #1079 (comment)

And a similar mention of label wrapping (which affects dividers too)

[owenatgov]

Cheers @colinrotherham! I double checked the history and couldn't find anything to suggest it needed to not exist. Just needs a second approval and a changelog entry and it can get merged.

[domoscargin]

Seconded!

@colinrotherham Do you want to add a changelog entry, or have us do it?

[colinrotherham]

Thanks @owenatgov @domoscargin

Would you and the team mind adding a changelog entry please?

Always feels strange writing my own thank you message