Update documentation Frontend version and enable the Tudor Crown

[querkmachine]

What

• Update the docs website to GOV.UK Frontend 4.8.0 or 5.1.0 (currently it's on 4.4.1, seemingly?)
• Enable the Tudor Crown in the website header on or after the 19th February.

Why

GDS properties are expected to update to using the new crown before 1st March.

Who needs to work on this

• Developer

Who needs to review this

• Developer
• Designer(?)

Done when

• The crown in the header has been updated
• Favicons, sharing icons and OpenGraph images have been updated