Autogenerate docs for gVisor global flags

gVisor flags can be configured from places other than directly on the command line. Examples are in `/etc/docker/daemon.json` or in containerd shim configuration.

It would be nice to have an autogenerated doc that documents gVisor's flags on the website so we can link to from Docker or containerd docs. That would make it easier for folks to discover what gVisor can do and how to to configure it.