Build the phase 2 Datum site

[chrisberridge]

@ronggur @felixwidjaja @AriaEdo @kaleygel

The phase 2 Datum site is now ready for build, to entirely replace the current site, and align seamlessly to the new Docs and Milo experiences.

Please build the new site using Olli, our design framework, which you can see here.

The template designs can be accessed here.

Please note: Once the Datum site is ready to go live, the Docs and Milo headers/navbars will need to be updated to use the same values, and ensure consistent navigation. See the Figma file.

I'll be componentising more of the templates as we go, but regardless they are ready to be built - ignore my movements in the file.

Finally, the content within these designs is due to be updated before launch, but let's get this built and then tweak.