Add step to check for MkDocs

[tupaschoal]

Better safe then sorry, I'm suggesting a step that just tries to run MkDocs in a simple action before actually trying to build

[peaceiris]

As I said at #16, it is better that we manage the version of dependencies by Pipenv.lock or requirements.txt. We can check the version by reading those files.

[tupaschoal]

My intent here was just to make sure it runs by trying a simple command. Could have been "help" or anything else.

If you think it doesn't make sense, feel free to close it.

[peaceiris]

I think that the current example is enough as just an example. Of course, users can expand their YAML settings as they like at their repository. Thank you for your suggestion.