Add PHP Optimizer guide to amp.dev

The [section on amp.dev that deals with the AMP Optimizer](https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/amp-optimizer-guide/?format=websites) has a guide on [how to use the Node.JS Optimizer version](https://amp.dev/documentation/guides-and-tutorials/optimize-and-measure/amp-optimizer-guide/node-amp-optimizer/?format=websites).

We should add a guide for the PHP version of the optimizer as well.