Rewrite/Improve Events guide

[FabianLars]

Also related: #486.

If possible we should look into improving it before the new docs are there. The guide causes too much confusion.

I will try to expand this list every time i see people having issues related to the guide or events.

• It emits events in the setup hook without some kind of condition. (New) users will therefore often encounter timing issues (emitting events before the js side starts to listen to them)
• Usage of unlisten() right after listening to it - idk if it's caused by mindless copy-paste but this was a problem in peoples apps wayyy too often.
• no mention of the need to unlisten in js when something like component unmounting happens.
• no docs on rust to rust communication -> talking about the trigger methods

[simonhyll]

Tracking for 2.0, think we're a bit too close to 2.0 for it to be worth fixing in 1.0. Will definitely need to keep these things in mind when writing a guide on events!