Icons documentation review and update

[foxintherain]

What

Content review and improvements of icons guide for all brands to identify improvements needed or gaps. Uses new structure, as defined in the styleguide audit.

Why

Icons documentation was created a while ago and needs updating following the styleguide audit.

Done when

• [x] Content has been reviewed
• [x] First Draft on design approach (best practices on icon usage)
• [x] First Draft on technical approach (based on frontend implimentation from @Sreilys and @sturobson
• [x] Check with UI designers on a snapshot of icons currently used
• [x] Document use cases and any ouliers / non chesive as a system
• [x] Document examples that do not follow our desired approach
• [x] Updates have been reviewed by UI design team
• [x] Updates have been reviewed by Elements team
• [x] Updates published

[amyhupe]

Currently blocked until we decide on our implementation - issue #72