Add article about message generation

[filipsch]

As you write chains of SCTs, every function 'contributes' to the feedback message that is generated in case the student made a mistake. All of these 'chunks' of feedback can be adjusted by setting arguments such as missing_msg, expand_msg and incorrect_msg.
An article should be added to pythonwhat that:

• explains in understandable terms how message generation works
• gives examples about it
• explains how you can customize messages generated by the SCT
• explains which automatically generated messages are not great and in which cases (e.g. when using has_equal_value() with set_context(), see Update incorrect_msg in has_expr when set_context/set_env used #317), and thus could use a custom message (for now).

Requested by @klmedeiros