Review coding standards documentation for CSS

[domoscargin]

What

The last significant updates to our coding standards for CSS were in 2023. We'll probably need to update them for our move to Sass modules, and it's a good opportunity to audit the doc in general.

Why

Ensure our contribution documentation is up to date.

Who needs to work on this

Developer, tech writer

Who needs to review this

Developer, tech writer

Done when

• Our CSS coding standards doc is up to date for our new styles standards.

[romaricpascal]

Let's make sure we document nuances introduced by our switch to @use, particularly:

• --internal modules to avoid unintentionally forwarding internal code
• using as * and keeping public API prefixed by govuk- to ensure members are named consistently when @used rather than depending on the namespace