Documentation design update

[paulrobertlloyd]

Following on from x-govuk/govuk-components#486, this PR:

• Fixes the logo in the header now that we need to support the new v5 way of doing things
• Uses the Masthead component styles from the GOVUK Prototype Components project
• Adds a modifier class to the header on the index page to account for the masthead
• Increases the overall width of the page container, and adds sub-navigation to all pages
• Moves content from the homepage onto the Getting started page (with some light editing)
• Uses the related navigation component styles from the GOVUK Prototype Components project for related information

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[peteryates]

Spot on, looks great. Thank you!