Add subcommands to container commands.

[stockholmux]

Is your feature request related to a problem? Please describe.

Container commands should list the sub-commands in the command reference.

Describe the solution you'd like

Documentation for container commands like CLUSTER doc pages should list commands like CLIENT CACHING, CLIENT CAPA, etc.

This could be achieved via a template change.

Describe alternatives you've considered

Maybe a tree like structure on the side bar.

Additional context

@zuiderkwast mentioned this in a meeting.