Add more service navigation examples

[querkmachine]

Adds a couple more examples of the service navigation component to the guidance page.

• A 'default' example that has navigation links and no service name.
  • This is probably the most basic intended use of the component—as a navigational element.
  • All existing examples included a service name, but this made it less clear that element landmarks are different if there isn't a service name.
• An example of the component alongside the GOV.UK header.
  • This shows how they're intended to be used together, as there were questions about whether the Service navigation should be within the <header> element or not.
  • This also visually demonstrates using the full-width border modifier class, which not all users were confident using from the written guidance.

[netlify]

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[CharlotteDowns]

This looks ace! I guess we won't see the changes come through from Fix service navigation mobile toggle spacing alphagov/govuk-frontend#5278 until a new version is released of Frontend? Either way I don't think it's worth delaying.

[calvin-lau-sig7]

Looks great!