Define layout guidelines for Codex docs

The best place seems to be section "Layout Guidelines" > "Grid"
The Container component can link out to Grid and Container and vice-versa

Guidelines

Open question

• We need to define how much imagery we want to support the guidelines with. Orientation point is @bmartinezcalvo blog post imagery on Grid a while ago
• Best possibly we would make Codex docs use the Grid mixins

Acceptance criteria (or Done)

• Document Grid guidelines (we will include them in Codex later)