Document `cog debug` command

Cog has a debug command that outputs the Dockerfile for the current cog model, but it's not documented.

cog debug > Dockerfile

It's not available in help:

$ cog --help
Containers for machine learning.
...
Available Commands:
  build       Build an image from cog.yaml
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  init        Configure your project for use with Cog
  login       Log in to Replicate Docker registry
  predict     Run a prediction
  push        Build and push model in current directory to a Docker registry
  run         Run a command inside a Docker environment

This command is mentioned in passing in the getting started guide, but is not present in any reference documentation.

See also:

• https://github.com/replicate/cog/issues/433

Start with docs/getting-started.md and the existing cog --help output to understand how commands are documented. Add reference documentation for cog debug, including that it outputs the Dockerfile for the current Cog model and the shown redirection example; done means the command is no longer documented only in passing.