Update the documentation for Flake8.
Update the documentation for Flake8.

This updates the docs for Flake8 to match the structure of the new tool
documentation. It includes a better description, more detailed
installation instructions, and additional configuration information.

Testing Done:
Built the docs. Checked for build errors, spelling errors, and bad links.

Reviewed at https://reviews.reviewboard.org/r/11788/

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