Provide example for localization routing

[jmarsh24]

The following page I have found on localization do not specify the recommended way to include localization routing into avo. I think this would improve the readability of the documentation.

for example

scope ":locale", locale: /#{I18n.available_locales.join("|")} do
  mount Avo::Engine, at: Avo.configuration.root_path
end

https://docs.avohq.io/3.0/i18n.html
https://docs.avohq.io/3.0/guides/multilingual-content.html#add-the-language-switcher