Cookbook feedback

[ChrisSoderberg-ONS]

Implemented changes from email feedback to the pkgdown site.

Also added clarification in the colours article about why departmental colour palettes are not supported

Changes NOT implemented:

• Suggestion to style afcharts as afcharts - this is not done in package documentation
• Suggestion to change circular link to the home page on https://best-practice-and-impact.github.io/afcharts - this page is also copied to README.md so the link should remain
• Suggestion to move contents in reference page - don't understand what this comment referred to
• Suggestion to add annotations to the first pie chart in the cookbook - this is just to demonstrate the pie chart so should be kept simple
• Suggestion to change the move the embedding charts section to near the top of cookbook->other customistations, it is more niche so it fits better further down