Write guidance around including a video in documetnation

## 📖 Missing or Out-of-Date Documentation

### Problem
The contributing guide has no information on embedding a video in documentation. We need to explain how!

Only current example is:

```

```

In the live [JavaScript tutorial](https://amp.dev/documentation/guides-and-tutorials/develop/custom-javascript-tutorial/?format=websites#congratulations!).

It's unclear how to size videos - the display on this tutorial is huge. cc/ @matthiasrohmer @sebastianbenz