Document how oEmbed works

**Description:**

Specifically:

1. Synapse bundles an official set of *providers*, and [allows you to provide your own](https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#oembed).
2. If there is a HIT, then Synapse can quickly do a oEmbed URL preview of the URL you sent.
3. If there is a MISS, we still [perform discovery](https://oembed.com/#section4) on the URL.
4. This occurs regardless of whether you configure `oembed` in your config.