Review the content in our typography technical documentation

[owenatgov]

What

Review the API documentation for our typography variables and typography mixins.

Why

So that it's clear to users how to interact with our typography API.

Part of https://github.com/alphagov/govuk-design-system/issues/2300

Who needs to know about this

Tech writers, developers

Done when

• [ ] The typography variables are reviewed
• [ ] The typography helpers are reviewed