Make converter support response key for test stories

[kalkbrennerei]

Closes #7034
Proposed changes:

• make MarkdownStoryReader::_parse_message use intent_response_key if a response key is used

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[degiz]

Great job testing that! 👍
One question - can we name these tests the same way? They are almost the same, the only difference is that one test normal stories, and the other - test stories.

Also - could you please add a changelog entry?

[tmbo]

OPT: style suggestion, use multiline string instead (needs import textwrap)

Suggested change

	assert content == (
	f'version: "{LATEST_TRAINING_DATA_FORMAT_VERSION}"\n'
	"stories:\n"
	"- story: id\n"
	" steps:\n"
	" - intent: out_of_scope/other\n"
	" user: |-\n"
	" hahaha\n"
	" - action: utter_out_of_scope/other\n"
	)
	assert content == textwrap.dedent(f"""
	version: "{LATEST_TRAINING_DATA_FORMAT_VERSION}"
	stories:
	- story: id
	steps:
	- intent: out_of_scope/other
	user: |-
	hahaha
	- action: utter_out_of_scope/other
	""")

[kalkbrennerei]

thank you!! since it already got merged, I will consider this for the next test I write!