cli: document `upgrade check` command usage

[jbw976]

After crossplane/crossplane#7451 is merged and we have the new upgrade check command in the release-1.20 branch, we should update the docs for v1.20 to help understand how to use it.

This could include:

• basic usage/help output on the CLI reference page: https://docs.crossplane.io/v1.20/cli/command-reference/
• mention in the v2 backwards compatibility page: https://docs.crossplane.io/latest/whats-new/#backward-compatibility
• mentions on v2 upgrade page: https://docs.crossplane.io/latest/guides/upgrade-to-crossplane-v2/