Complete JavaDocs for the API

[Jonathing]

This PR adds complete JavaDocs to EventBus 7. These JavaDocs include explanations for type usage, use cases for the various characteristics, and non-exhaustive example snippets.

Additionally, this PR adds JetBrains annotations. The annotations in question are not @Nullable or @NotNull, but rather documentation annotations such as the ones in ApiStatus and IDE support annotations such as @Contract.

The JavaDocs have been written to be best presented within a JavaDocs website, which would theoretically be on the MinecraftForge domain (i.e. javadocs.minecraftforge.net), but they are also included as usual in the javadoc.jar file built by the project.

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Depends on the following PRs being addressed or merged first:

• [x] #73
• [x] #76
• [x] #77

[Jonathing]

I plan on being finished this evening (Eastern Daylight Time). I've created this draft PR as a way to pace myself and view the work on GitHub as I complete it.

[Jonathing]

Update: In CLASSIC Jonathan style, I've bitten off slightly more than I can chew, and I am about to fall asleep as I type this. However, I am more than halfway there. As you can see from the handful of the new issues and PRs I've made, I've acquainted myself with the API of EventBus 7 quite well.

I'll push what I have and continue tomorrow. Like with everything else I've been doing with the toolchain, I want these docs to be held to the highest standard to which I can provide. So far, all that's left (aside from maybe a few more example snippets in existing packages) are the event and listener packages.