Add tutorial about Gatherers

[GrimReaper3223]

Hello! How are you?

I would like to make an important suggestion about implementing a tutorial talking about the Gatherers API.

I know it's still a preview feature, but I've noticed in some videos that it's pretty useful!
Unfortunately, there are no pages and forums that explain content as well and in text as here on dev.java.

Thank you in advance for your understanding and contact 🖤

Best regards, Deiv.

[danthe1st]

@GrimReaper3223
I think you accidentially created this discussion twice, see #133