Proposal: new structure for example pages

This proposal is for the SCSS structure used for example pages to be built around the readability of the resulting CSS. This includes favouring readability over any optimisations that would normally be applied to reduce duplication.

To comply, the resulting CSS should meet these requirements:

1. The CSS for a page shouldn't contain CSS not used on the page
2. documentation about the way CSS is applied should be in comments in the CSS, nowhere else
3. SCSS partials should be single-purpose to satisfy point 1. & so their @import statements explain what they do

Note: point 1. is to help developers so when they inspect the CSS to see what is needed to make that pattern they aren't confused by code used somewhere else.