[feature] Doc generation command

[texastoland]

Purely for tracking WIP. Related to:

• Adding basic doc generation functionality #60
• Generate documentation for std commands nushell.github.io#1210
• generate and check documentation amtoine/nu-git-manager#128
• https://discord.com/channels/601130461678272522/729071784321876040/1199113819386028102

Description

The only thing to add is it'd greatly help if the help command returned structured data (maybe help --detailed for symmetry with describe). Not creating that issue because the core team is in this repo and I'm not ready to implement it myself 😅