Added initial "Site Tree" implementation

[nwittwer]

UX:

1. Add a first URL
2. Add sub-page paths (e.g. "/about")
3. Easily navigate to any of those sub-page routes, screenshot all screens sizes for that page, and assign a status

Problems to solve:

1. Easily jump to another page of a website; typically need to build page-by-page and want to review them often
2. Keep track of the status of page implementations