Add docs for `convert composition-environment` command

### 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 https://github.com/crossplane/docs/pull/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.
```