std doc page

[NotTheDr01ds]

• Adds some much-needed detail to the std doc
• Removes the link to the Nushell GitHub repo - The README there is out of date anyway, and the doc should be rendered by the SSG (VuePress) rather than in a GitHub Markdown file whenever possible.
• Might be controversial, but I note the performance issues that prevent new commands from being added to std at the moment. I think it's a good explanation give the current state, and it doesn't come across too harsh, hopefully. I can remove it if needed.

[fdncred]

Thanks