Create new `Titlepiece` component 🎡

[frederickobrien]

What does this change?

The first step towards adding a fully fledged Titlepiece component for the new-look site masthead. This PR creates a new component and focuses on the design, with the appearance adjusting as desired across multiple breakpoints.

Implementation build on a CSS Grid approach modelled by @mxdvl during the Basecamp project (for reference), with different parts of the titlepiece positioned on a shifting grid.

Companion to #11190. The contents of this component will be kept out of public view by the same 0% test that the masthead as a whole is sitting behind.

Not covered in this PR but will need to be done in follow-ups:

• [ ] Veggie burger menu on mobile
• [ ] Tracking on the edition switcher
• [ ] Pillars nav dropdown on click
• [ ] Generating the pillar and subnav links using the nav prop (this should also capture highlighting section colours depending on where the reader is)
• [ ] Side/carousel scrolling for the subnav on smaller devices

Plus a bunch of silly mistakes/omissions I've no doubt made.

Why?

As part of the ongoing 'Fairground' project to revamp the homepage, we're taking an incremental approach to the new-look masthead. This PR should lay a base on which we can add further functionality and best practice.

Screenshots

Screen width
mobile
tablet
wide