Create new.carbondesignsystem.com sandboxes landing page

[tay1orjones]

A centralized landing page should list all the sandboxes available within carbon-design-system/sandboxes

Why

It would be particularly helpful for (bug) issue templates. In the templates we have a field for a reproduction link, and we link out to a couple sandboxes but those links easily become out of date, and don't provide authors with sandbox options pertinent to their environment (react 16/17/18, carbon v10/v11, typescript/javascript, etc.)

How

This landing page could list out sandboxes for not just @carbon/react but any other framework variant: @carbon/web-components, carbon-components-angular, carbon-components-svelte, etc.

"Pattern and asset libraries" could have their own section as well

Ideally we'd just reuse the existing readme.md to generate a page hosted via github pages on this repo. From there a new.carbondesignsytem.com subdomain can be configured to point to the github pages page.