Docs: add GC user docs

[IanButterworth]

Closes #54565

Initially generated using Claude 4, with review and tidyup.

@d-netto or others on the GC team, please feel free to push to this directly.

[adienes]

Suggested change

	* **Mark threads**: Used during the mark phase to trace object references (default: same as compute threads (thread 1 is shared))
	* **Mark threads**: Used during the mark phase to trace object references (default: 1, which is shared with the compute thread if there is only one, otherwise half the number of compute threads)

the nested parenthetical here reads a bit odd, and I'm not sure how to square the information with https://docs.julialang.org/en/v1/manual/multi-threading/#Multiple-GC-Threads stating that #mark threads = 1/2 #compute threads

[IanButterworth]

Maybe the point I was trying to highlight isn't important, but the first gc mark thread is the same thread as thread 1.

% ./julia -t6 -q
julia> versioninfo()
...
Threads: 6 default, 1 interactive, 6 GC (on 6 virtual cores)

julia> Threads.maxthreadid()
12

(not 13)

I've been confused by that before. But maybe that's too niche.

[adienes]

now I'm confused even more 😁, if there are 6 worker threads and 6 GC threads, how is that "half the number of compute threads" ? are the original docs wrong?

[IanButterworth]

Oh right.. good point

[d-netto]

Yes, the default number of GC threads is now --threads.

[d-netto]

#53608

[IanButterworth]

Ok. I added a docs fix, not not sure if I found every instance. It's a hard thing to search for.