Generate documentation for std commands

[texastoland]

The std module is only passingly mentioned in a placeholder page. Currently the only way to learn about it is introspection or reading https://github.com/nushell/nushell/blob/main/crates/nu-std/std/mod.nu (how I discovered it). The testing submodule is "deprecated" but that's a separate issue 👈🏼 Already moved to a crate. I don't have any reservation about alpha development stage since Nushell isn't 1.0 yet. Not opposed to a banner indicating it's less stable than other APIs.