Adjust nested list spacing

[pufferfish101007]

Description

Adds additional spacing at the beginning of nested lists, and removes unnecessary spacing at the end of them. Resolves #430 by a solution which is effectively https://github.com/nhsuk/nhsuk-frontend/issues/430#issuecomment-489084964. Also adds an example page, /components/nested-lists/index.html, to demonstrate/test that this does indeed work (I wasn't sure if this was necessary/the best way to do this but did it anyway).

Before: After:

Checklist

• [ ] Tested against our testing policy (Resolution, Browser & Accessibility)
• [x] Follows our coding standards and style guide
• [x] CHANGELOG entry

Regarding testing: I have tested against all expected resolutions, but only in Chrome 120, Firefox 122 & Edge 121 (all on Windows); I don't have access to any of the other browsers/devices currently. I also don't currently have access to any of the assistive technology support; I'd hope that what is essentially a CSS-only change wouldn't affect accessibility. Apologies for this lack of testing. I also tried running the backstop tests but it didn't really work (see https://github.com/nhsuk/nhsuk-frontend/pull/920#issuecomment-1929200064); all backstop tests that fail also fail on main

[pufferfish101007]

I carried out the backstop tests and got an awful lot of failed tests. A lot of these were because the reference image was missing? However I get roughly the same fails on the main branch (55 fails on this branch, 56 on main) so I don't think this introduces any regressions (that are tested for); there's probably a problem with my setup.