Update developer documentation for Active Context

Update documentation in gems/gitlab-active-context/doc/ so developers understand how to add new queues - specifically info about the fact that the queue class must be defined in the initializer) The following discussion from !200170 (merged) should be addressed:

• @terrichu started a discussion:

  nitpick (non-blocking): I saw the queue_classes has a default of [] if not defined, but I think queue_classes should be defined in the initializer here, something like this

    config.logger = ::Gitlab::ActiveContext::Logger.build
    config.queue_classes = []
    config.queue_classes.concat([::Ai::ActiveContext::Queues::Code]) if Gitlab.ee?

  I won't block on this MR, but do recommend a quick follow up to:

  • make that change here
  • update the developer documentation (in gems/gitlab-active-context/doc/) with information that this file must be updated when new queue classes are introduced