Improve plugin documentation

[Major-]

This is an epic that involves:

• Introductory information ("what are plugins and why do they exist")
• Information for developing, testing, and releasing plugins
• In-code documentation for plugin APIs.

See #330, #392.

[Arham4]

Writing plugins was of complete ease for shops. I will say, however, that the error messages it prints out should be more clear.

For instance, I was stuck on why I got a KotlinNullPointerException. It turned out it was due to adding an item name that didn't exist. After some debugging by modifying the DSL, the item at fault was "Kharidian headpiece" (despite copying from the wiki), only to find it is actually spelled "Karadian headpiece" in the source. If the error was more clear to that it being a spelling error (perhaps InvalidItemException or something?), it would have saved a lot of time. I had the same thing happen to waterskins and had to rinse and repeat the process.

Albeit this can be resolved by putting IDs for all items, that was not the intention behind the DSL.

[garyttierney]

Expanding on above, I'd like to also eventually provide editor support in IntelliJ for this sort of thing. An Apollo plugin could load the users local cache and do error-checking on annotated methods to determine if a given string identifier is a valid unique entity, ambiguous, or not valid at all.