Add link to 10up PHPCS standards

[benlk]

Is your enhancement related to a problem? Please describe.

This website contains extensive documentation of PHP coding standards, but doesn't link to our automated tool for testing code against those standards: https://github.com/10up/phpcs-composer

We should add a link to that at the top of https://10up.github.io/Engineering-Best-Practices/php/

Designs

No response

Describe alternatives you've considered

No response

Code of Conduct

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