Add initial markup and styling

This starts the process and will let us bang on the hood.

Still to come:

• Battle test it against real content, see how it survives
• Cross-browser test
• Anything to do with the Organization Readiness Tool

Let's chat to talk about implementing this with real navigation and all that!

https://code.librehq.com/ots/clients/lfc/torque-tracker/-/issues/305