Add no_subcommands option

[tahv]

This PR add the no_subcommands options to skip the recursive generation.
It partly addresses #54 and is inspired by sphinx-click :nested: option.

The idea is to have more control on the toctree depth. Ideally, i'd like to have all my commands to have the same heading.
The uv CLI Reference doc is a good example of what i'd like to achieve with this.

I first added a :nested: option that was True by default, but seeing all the other options default to False,
i changed it to :no_subcommands:, but it could also be an enum :nested: full | none (like in sphinx-click).

With subcommands:

Without subcommands: