Block out some element page sections

[justinfagnani]

Just adds a basic grid for header / sidebar / content and rough not-quite-approximate styling of headers to start to see an overall layout.

Lots of hard-coded values, as we've had so far. All the styling will need a audit when we really start making it look like forthcoming designs.

[github-actions]

A live preview of this PR will be available at the URL below and will update on each commit. =
The build takes ~5-10 minutes, and will 404 until finished.

https://pr1415---site-khswqo4xea-wl.a.run.app/

Warning
Until our Cloud Run project is public, only authorized users can view the above URL.
The easiest way to view it authenticated is to run the following proxy command and visit http://localhost:5453

(gcloud beta run services proxy --project=webcomponents-org-test --region=us-west2 --tag=pr1415 --port=5453 site \
& gcloud beta run services proxy --project=webcomponents-org-test --region=us-west2 --tag=pr1415 --port=6453 catalog)

[justinfagnani]

We need to send version and dist tag info in the page data.