[EPIC] New docs site

[joelanman]

What

Use a new tech stack and design for the docs site

Why

The current stack and design is not ideal:

• it is built on the Prototype Kit, we tell people not to use it for production
• the code is inconsistent and hard to work on
• the design isn't very consistent with other GDS product sites
• the information architecture could probably be improved
• users would benefit from a search

Notes

I believe the Design System team are interested in investigating Eleventy, we could work with them

Related issues

• Code blocks are often cropped at the 2/3 width #154
• The docs website requires a server-side rendering app #102
• Can we automatically generate the "Tutorials and examples" links? #131
• Header nav doesn't indicate active page #111
• Documentation pages don't automatically refresh when developing locally #93
• GOV.UK Design System depends on Prototype Kit website previews of templates  #100

[joelanman]

could also use vanilla express