Fix some formatting, refactor external links (docs)

[bittner]

Follow-up on #130. Makes minor corrections on the configuration chapter of our documentation.

@kingbuzzman, can you verify whether the link to the behave source code is appropriate? Can something be added or changed to explain the role of behave's runner even more clearly?

[bittner]

BTW, how would someone build a customer runner? Can we give some hints in the documentation? Would that make sense?

[kingbuzzman]

A couple of reasons come to mind:
Django

• Change the TestCase and replace it with something custom (https://github.com/django/django/blob/stable/4.0.x/django/test/runner.py#L562)
• Set custom flags by default (https://github.com/django/django/blob/stable/4.0.x/django/test/runner.py#L576-L582)

Behave

• Change the way discovery of *.feature/step files work https://github.com/behave/behave/blob/v1.2.7.dev2/behave/runner.py#L743

[bittner]

Thanks for your input, @kingbuzzman! The result is now online at behave-django.readthedocs.io. Please, take a look! 👀