Fix make_docs.nu, adding generate docs for plugin commands support

[hustcer]

We need to register the plugins first and then generate the docs for related plugin commands.

BTW: It's weird that if you run nu make_docs.nu there will be an error output:

Error: nu::shell::plugin_failed_to_load

  × Plugin failed to load: Plugin file not found

However, all the command docs have been generated successfully.

[fdncred]

Thanks for all the great work you do on our docs!