Add a Data Catalogue for easy download of all LEEDS 2023 data

[taz-chiles]

We have agreed to add a basic Data page/ section of the website that includes straightforward download links in CSV format, so that people can visit the site, go straight to the data and download the relevant data.

This might simply be a page on the data.leeds2023.co.uk/data url, with a Data navigation menu item that links to a simple page that lists each Metrics page and provides a "Download the data" link with a CSV download.

@gilesdring we might want to repurpose the page you created to monitor the data pipelines on the same url?