Guidance: searchMode prop on ic-search-bar

[GCHQ-Developer-741]

What is the purpose of the work?

A new prop has been added to ic-search-bar which allows for custom values to be submitted at search. This prop takes the form of searchMode with two values: navigation (existing behaviour) and query. Guidance needs to be added to the site explaining this new prop and how it changes the search bar behaviour

Why do we need it?

So that users aren't confused when using this prop and seeing different results from what they may be used to seeing