Import AMP Shadow Doc API documentation into amp.dev

## 📖 Missing or out-of-date documentation

**Describe the content that is missing or should be up dated**

The [AMP Shadow Doc API](https://github.com/ampproject/amphtml/blob/master/spec/amp-shadow-doc.md#shadow-doc-api) documentation deserves a home on amp.dev. There exists [partial documentation](https://amp.dev/documentation/guides-and-tutorials/integrate/amp-in-pwa/), but it is more of a "how to get started" than a resource for developers.

**Which part of the site would that content live at?**

The most suitable documentation category would be "Runtimes", but I'm not sure what else would live in this new category. I'm not passionate about its location, as long as it's easier to find than the GitHub spec document.

**What's the type of content?**

API documentation (moderately technical).