Remove Imagery documentation from the styleguide

[foxintherain]

What

Following the styleguide audit, Imagery should be removed.

Why

• Only Springer Nature has content for this at present as it seems a bit out of date
• We don’t use imagery very often as its usually for marketing / landing pages
• Our use of photography and may be about to change in the near future with the new direction for the brand

Done when

• [ ] Content has been removed