Review pkgdown accessibility

[alice-hannah]

• See pkgdown documentation on known accessibility issues.
• Accessibility statement
• Review Sa11y feedback
• Should alt text be shown under chart instead of in alt tag?
• Should chart titles etc. be in main body of page instead of embedded in chart image (see afcharts implementation)?
• Should all examples avoid use of legends? This is discouraged in guidance and alternatives demonstrated in annotations section.
  • If keeping legends, discuss how to change order to match order of data in chart? (https://stackoverflow.com/questions/69541579/ggplot2-reorder-items-in-a-legend)