mkdocs add conflict warning for mkdocs-material

[clintonsteiner]

• [X ] Have you followed the guidelines for contributing?
• [ X] Have you ensured that your commits follow the commit style guide?
• [ X] Have you checked that there aren't other open pull requests for the same formula update/change?
• [ X] Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [X ] Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• [ X] Does your build pass brew audit --strict <formula> (after doing HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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