Add information for plugin developers and paperweight userdev

[Phoenix616]

It would be good if the docs would include information for people that want to start developing plugins against the paper-api. Things that it should (imo) include:

• Maven/gradle info as well as common IDE setup information (e.g. Minecraft Development IntelliJ plugin)
• Links to general plugin documentation on the Bukkit and Spigot wikis
• Information regarding changed/added API and libraries (e.g. adventure). Ideally this would be pretty detailed but just having a general overview (e.g. important events and api groups like pre spawn and player profile) should be a big help for the start

[MiniDigger]

I like.

Slightly related, simple is working on exposing the maven info better https://github.com/PaperMC/papermc.io/pull/72

[jacobsandersen]

Next thing I am going to do is write these pages