Update icons documentation

[RobTobias123]

Outcome

An updated documentation page in Foundations / Icons that shows the latest grid pattern in images and is more specific about the use of colour and how it is applied (eg. created in black, colour applied at parent level and consequences to bear in mind if the brand colours are updated etc)

Scope

• Read through and update text where appropriate (colours, grid, SVG etc)
• Pick up latest grid being used for Nucleus icons (see BG icons library, formed at brand refresh) and apply to docs page.
• Check alignment of Amir's instructions on that Figma file. https://www.figma.com/file/aQNgr9LbCdeCfTnJKo2sEp/British-Gas-Icons?type=design&node-id=1%3A4991&mode=design&t=40AhlXS3YcGeZVlb-1
• Check and update if necessary the Figma UI kit library version fo the grid and configuration instructions and publish
• Create and update images with latest grid overlay.
• Approval
• Merge
• Publish

[RobTobias123]

Supersedes and includes #201 (now removed)