Add example for belongs_to/has_many factory

What does this MR do and why?

This MR adds an example for creating a factory for a belongs_to/has_many relationship because it's not obvious how to implement this in a way that doesn't cause a rubocop failure.

For example, I first tried the following:

FactoryBot.define do
  factory :car do
    after(:create) do |car, evaluator|
      create_list(:wheel, 2, car: car)
    end
  end
end

FactoryBot.define do
  factory :wheel do
    car { association :car }
  end
end

but this caused a rubocop error:

Prefer inline `association` over `create_list`. 
See https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#factories [RSpec/FactoryBot/StrategyInCallback]

If you visit the above link, it explains:

Prefer implicit, explicit, or inline associations over create / build for association setup in callbacks. See issue #262624 for further context.

If you visit the implicit/explicit/inline links, it's not immediately obvious how to set up a factory for a belongs_to/has_many relationship.

If you look through other factories in ee/spec/factories/ for inspiration, you'll notice that most of them are using the disallowed pattern after(:create) { |obj| create_list(:factory, relation: obj) }, however, these definitions are able to avoid the rubocop errors because they’ve all been added as exceptions in .rubocop_todo/rspec/factory_bot/strategy_in_callback.yml.

The actual solution for this problem is to use the instance method which is explained in the factorybot docs for has_and_belongs_to_many associations, but this is not at all obvious.

Because of the difficulty I had figuring this out, I thought it would be a useful addition to the documentation.

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