Initial quarto report with charts

[francisbarton]

Closes #19, #20 and #22.

build_datasets.R now brings everything together to create a single dataset saved as .rds.
This provides all the data required for the quarto report.

This PR provides:

• a tidied up dataset build process
• a ggplot2 theme combining SU theme colours and bootswatch colours
• some helper functions to increase readability of data pipelines prior to chart creation (more work to do on this).
• initial charts with summary data for national contacts and single-ICB contacts

Further quarto development will take place in a new branch, including development of further helper functions for pre-plot pipelines.