Document new serializers

Like: https://github.com/rstudio/apis-plumber/pull/20/files

```
`@serializer rds` | `application/octet-stream` | `base::serialize(version="2")`
`@serializer rds3` | `application/octet-stream` | `base::serialize(version="3")`
```

* csv
* yaml
etc.

(wait for pkgdown to be merged)