feat(velocity): document velocity-plugin.json file

[Strokkur424]

closes #731

I actually decided against outright removing mentions of the AP, as it is useful for beginners and honestly doesn't really hurt most people. But having the actual file documented seems like a good thing.

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[Timongcraft]

I think it should either be mentioned what the name and ID are actually for, because most logging just uses the ID so either that should be refactored, or this behaviour be documented. (The name is also mostly in-game facing)
I'm also not sure why you omitted parts of the spec in your example, because I think the author/url field is a good source indicator, that should be recommended.
Also I think this page could be a bit more compact, like putting a required indicator in your example or in the header of the entries itself and/or not have these giant spaces.
Lastly IMO the page should be under the "Creating your first plugin" page or be a sub-page and be mentioned in the:
https://papermc-feat-velocity-plugin.papermc-docs.pages.dev/velocity/dev/creating-your-first-plugin/#setting-up-the-dependency section, not just the file itself.