Adding basic doc generation functionality

[TWSiO]

Description

This change adds basic documentation generation functionality to nupm.

Using the current package's code, or paths to other modules it will generate a directory of markdown files based on the internal metadata about the modules and commands (data from scope).

The majority of the code is shamelessly stolen from @amtoine's nu-git-manager toolkit script and modified.

There's a lot of basic functionality missing that I'd like to add like generating HTML and documenting plugins, but this is a good first step.

I also added the generated documentation for nupm itself because it seemed appropriate.

[kubouch]

Before adding nupm doc and committing to this particular style, we should first have a discussion about how a doc autogeneration would work in Nushell in general. Then add support for it in nupm. So far, we haven't discussed this thoroughly in the core team. I'm not saying this is a bad doc style, and we might as well use it as a starting point for the discussion, but I don't feel comfortable committing it to nupm yet because it might as well change significantly (e.g., we could add this functionality to Nushell itself, or use a separate program to do the doc autogeneration). One approach would be to create a nupm experimental subcommand group and move the doc there without commitment, just to test it out.

[amtoine]

marking as DRAFT for now