Add documentation to plytool interface

[tiaanl]

Currently it is hard to use the plytool without reading the documentation, which is also lacking in details about the commands available.

I propose adding documentation in the feedback while using the plytool like other cli based tools. E.g.

> plytool
Usage: plytool <command> <subcommand> [<arguments>]

Available commands:
folder - Manages folders in the workspace.
generate - Generates/updates the build files.

Arguments:
--verbose - Increase detail of log output.
...

This will improve usability of the tool by a lot.

If this is something that makes sense for the future of the tool, I'd be willing to help contribute this feature.

[preshing]

That would be great. Since the output could be quite long, I was thinking of separating it into multiple help commands. Sort of like the Perforce command line.

plytool help module
plytool help folder
plytool help generate

[preshing]

Thanks for the structure. I'm in the process of extending some PlyTool commands and will add proper descriptions after that.