update mkdocs.yml
update mkdocs.yml

A few changes:

- Update website domain from u-root.org to u-root.tk
- Use the default mkdocs theme for simplicity
- Make website a single page, index.md
- Add a copyright

Signed-off-by: Ryan O'Leary <32139beaa9c574695821e85473b09a4e33316467@google.com>

hugelgupf/u-root,u-root/u-root,rminnich/u-root,hugelgupf/u-root,hugelgupf/u-root,u-root/u-root,rminnich/u-root,GanShun/u-root,rminnich/u-root,hugelgupf/u-root,GanShun/u-root,GanShun/u-root,rminnich/u-root,u-root/u-root