Major changes: design guide content refactor

[klepas]

Description

This branch holds all the SASS comment documentation changes.

Most of the edits here are deletions, where documentation and guidance that previously was entirely contained within the SASS partials have been moved into the Design Guide, as markdown content.

These changes make the current KSS build system redundant.

We may also want to pursue further tidy-ups — expunge anything else that is no longer now being used, but that can continue after.

Definition of Done

• [x] Content/documentation reviewed by Julian or someone in the Content Team
• [x] Code reviewed by one of the core developers
• [x] Acceptance Testing
  • [x] HTML5 validation (CircleCI)
  • [x] Accessibility testing & WCAG2 compliance (npm test)
• [ ] Stakeholder/PO review
• [x] CHANGELOG updated

[klepas]

No new errors from the npm test results.

[klepas]

@dominikwilkowski Done. :)