Introduce a docs FAQ section

[mhauru]

• "Why is my model slow?" Ref: Better performance guidelines and support for user #2414
• "I changed one line of my model and now it's so much slower; why?" Ref: Better performance guidelines and support for user #2414 + how to investigate type-instabilities.
• "Which automatic differentiation backend should I use?" Ref: Better performance guidelines and support for user #2414 and specifically Automate choice of AD backend #2417
• "Why is this variable being treated as random instead of observed?" This is a question we get a lot. It's usually because it's not clear to the user that one can only manipulate expressions that explicitly occurs on the LHS of a ~, e.g. x ~ filldist(Normal(), 2) is in the model, but then they try do to condition(model, @varname(x[2]) => 1.0) or something.
• "How do I implement a sampler for a Turing.jl model?" This should just reference a tutorial exactly on this.
• "Can I use parallelism / threads in my model?" Yes, but we should point the user explicitly to where this is demonstrated and what to be aware of.

More?

[torfjelde]

Added some Qs + As-ish here @mhauru 👍