update plugins.md in contributor-book

[BrianLondon]

Updated plugins.md in the Contributor Book to reflect the current API. The page in its current form appears badly out of date. While the Contributor Book is not linked from within the main docs website, https://www.nushell.sh/contributor-book/plugins.html shows up at the top of web searches for terms like "nushell plugin tutorial" and at least confused me.

I know there are Rust and Python example plugins in the Nushell repo, but I appreciated the simplicity of the example that was in the Contributor Book along with the narrative walk through. I just wished it had reflected the current API. This PR aims to fix that.

[fdncred]

I see this as a step forward but some of the old parts are still wrong. I also have not checked the code to ensure it works. However, I don't think that should stop us from landing this.

[BrianLondon]

Thanks @fdncred. If you want to call out any areas of the narrative (I'm guessing somewhere in the first two sections) that are still out of date, I'd be happy to take another pass through the text.

[fdncred]

Hey @BrianLondon Thanks for the offer.

1. it's not really JSON-RPC anymore, it's just a json or msgpack communication protocol. json works with python but msgpack is many times faster. the hope is that any language could write a plugin with json or msgpack, but i'm not sure i've seen anything by python and rust since we've moved over to this new format.
2. the parts with config set blah aren't valid any longer because config set isn't a valid command
3. the c# reference is outdated and needs to be removed from the bottom.