guidance: Review and update prototype guidance

[htmlandbacon]

What

We should review our prototype guidance #347 and make sure it still works.

We may need to understand if this is something that has been changed in the prototype kit or we'd recommend using the unbranded.njk layout

Why

Raising this as our font guidance doesn't seem to work in the latest version of the prototype kit (@mitz-lad has also checked this).

Done when

• Guidance has been reviewed, updated and tested

Outcomes

Updated guidance on setting a font in the prototype kit

[simoneduca]

The new guidance is here #661