Add a guide explaining CSS optimization strategies

## 📖 Missing or out-of-date documentation

We should add a guide explaining strategies on how to best optimize CSS for AMP to avoid hitting the limits. Things we should cover:

* css compressors (csso, cssnano, ...)
* modularizing css to only pull in what's needed
* split animations into separate keyframes block (AMP Optimizer does this automatically)
* rename class names
* removing unused CSS and potential pitfalls when using amp-bind or amp-list.

//cc @CrystalOnScript