Move plugin tutorial into it's own opam package

[Alizter]

I think indeed we can close this, @Alizter maybe we should move the plugin tutorial to its own opam package (as to have a finer control over building) ?

Originally posted by @ejgallego in #15718 (comment)

[ejgallego]

I understand that means you think it is a good idea?

[Alizter]

I think it is a good idea. I don't think it is something we want to ship in the main coq opam pacakge anway. So along with CoqIDE we should ship it in its own package.

[Alizter]

The plugin tutorial absolutely needs to be updated somewhat too.