Simplify MkDocs example

[tupaschoal]

This might be a very näive suggestion, but using a requirements.txt for something as simple as using MkDocs seems like an overkill. What do you think?

[peaceiris]

In general, we should lock the version of dependencies for stable CI/CD execution using pip freeze or pipenv lock. I don't think that this change is better.

[tupaschoal]

I figured for MkDocs it might not be needed, but I get your point. Closing PR, thanks

[peaceiris]

Thank you.