Support subcommands

From readme:

> Note: there's no support for subcommands, create an issue if you need it.

So, here we go, creaing an issue :)

This is a very valuable crate since clap v4 has degraded help screen aesthetics compared to v3. Thanks for publishing!

Start with the README note that subcommands are unsupported and inspect the clap-help crate's current help-rendering entry points. Define how subcommands should appear and verify that help output supports them without regressing existing command help.

Written by the indexing model from the issue text.