Add mapping guidance and examples

[alice-hannah]

Aim: To support analysts to create their own static maps in R.

Suggested guidance topics:

• Where to find common shape files (inc. low vs. high water mark)
• Transforming and simplifying geometries
• Copyright
• Use of colour
• Selecting break points
• Alternative formats (e.g. alt text, data table)
• Links to other guidance (e.g. SG design system)

Possible helper functions:

• ggplot theme
• copyright statement

An unfinished attempt at this content is available: https://github.com/alice-hannah/maps-draft