Add docs for convert composition-environment command

[jbw976]

What's Missing?

The https://docs.crossplane.io/master/cli/command-reference/#beta-convert documentation is missing the new convert composition-environment command.

New content relevant to this area was added to the concepts Environment Configs page with #824, but we missed updating the CLI section with the new command to help folks migrate their environment config usage.

From crossplane -h:

  beta convert composition-environment
      Convert a Pipeline Composition to use function-environment-configs.