added more information on gc and rooting

[Narfinger]

This adds more information on Gc and rooting.

[Narfinger]

Thanks for the comments! Currently I can't look at the formatting and how it looks in compiled form that is why it is in draft mode (and so other people can check the correctness).
If somebody could contribute a small section how to use the rooted1 macro that would also be awesome.

[jdm]

In-progress review because I need to think more about this, and also sleep. We jump from a very simplistic view of the GC (it can happen anywhere! at any time!) to suddenly passing around a mutable pointer to the context and talking about unrooted values. I think we probably want to introduce the idea of &mut JSContext much earlier in this documentation, possibly in the first example.

[Narfinger]

Ok github screwed up a bit so I hope I got all the suggestions in.
I can think about a bit more how to include the JSContext stuff.

But I feel like it might not make sense to have it at the start. The start should be imho about why we need Gc anyway, What the rooted types are and how crown fits in all this.

Then (probably before performance subsection) we can introduce JSContext. Something like: "But Gc actually only runs when we do specific spidermonkey apis and having CanGC everywhere is hazardous".

[Narfinger]

Ok I added, hopefully, a better transition from rooting to CanGc and then to JSContext.

[jdm]

The flow makes a lot more sense to me now! I've made a bunch of editorial suggestions, but we should be good to merge this soon!