Create provenanced command for generating provenanced documentation.

[SpicyLemon]

Summary

Add a docgen sub-command to provenanced that is hidden and will generate documentation for the provenanced command.

Problem Definition

As a new or prospective user, I want to read about the provenanced commands before installing the program so that it's easier to learn about it.

As a maintainer of provenanced, I want to auto-generate documentation for use of provenanced so that I don't have to spend time thinking about it, and it's done.

Proposal

Add a new docgen sub-command to provenanced.

• It should have Hidden: true.
• It should take in a directory path as an argument.
• It should have flags for the various output types: --markdown, --yaml, --rest, --manpage. It'd be nice to have a --json option too, but since the cobra docgen stuff doesn't have that option, and writes the file(s) on its own, it might not be worth the effort.
• If none of the output type flags are provided, just do markdown. If multiple are provided, do each provided.
• It should use the built-in cobra docgen stuff: https://github.com/spf13/cobra/tree/main/doc

The command would be invoked like this:

$ provenanced docgen '/tmp' --yaml --markdown

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