Add example of linking to core supporting pages from footer

[cjrace]

Is your feature request related to a problem? Please describe.
For things like the accessibility statement, cookies and support pages we should ideally link from the footer to keep the dashboard cleaner

Describe the solution you'd like
An example of doing this in the template

Describe alternatives you've considered
None

Additional context
Will need either a custom footer function or an update to the one in the shinyGovstyle package

Can use a hidden tabset linked to from action links in the footer and gov uk backlinks on the pages