Update readme with new docs.
Update readme with new docs.

- Rearrange the docs into sections on using Github or Git locally.
- Remove the troubleshooting FAQ since we build using Docker.

google/gvisor-website,google/gvisor,google/gvisor,google/gvisor-website,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor