[DOCS] add multithreading discussion in performance tips

[Datseris]

The points from this post as useful to have in the docs:

https://discourse.julialang.org/t/multi-threads-error-undefreferror-in-simple-abm-with-agents-jl/110900/19

mainly the locking of dictionaries, or the usage of a :dead field for later mass removal, or the usage of a thread safe dictionary for agent container.

(this means that we need to change the agent container types in the source code from Dict to AbstrcactDict and from Vector to AbstractVector)

[Tortar]

I would remove the locking of dictionaries because it is not specific enough in my opinion (clearly we can say that this is the methodology a thread safe dictionary in ThreadSafeDict.jl uses though), I like the other suggestions but the thread safe dictionary one should be tested if it actually works at all before proposing it

[Tortar]

In any case cool ideas !

[Tortar]

I think that https://discourse.julialang.org/t/multi-threads-error-undefreferror-in-simple-abm-with-agents-jl/110900/21?u=tortar proves the viability of a thread-safe dict!