Add a standard component / example for navigation in a side bar

[cjrace]

Slightly running ahead of GDS here as they don't have one yet, and are in their discovery phase, but it's a thing that is already being used widely and it would be good to set a clear example for good navigation in R Shiny.

Similar to how UKHSA have done in their data dashboard, we should provide guidance on how to create accessible left hand navigation links, either through examples or functions we can add into the package.

Full desktop view:

Mobile view:

[cjrace]

We've done a version using tabs at DfE, but this was pulled up as potentially confusing as tabs are usually listed horizontally rather than vertically. Ideally we'd set this up using link text rather than tabs.