Some Element Call setup resources say that `/.well-known/element/element.json` should be served on the base domain
and should contain content like this:

```json
{
	"call": {
		"widget_url": "https://call.element.example.com"
	}
}
```

We were already generating the file via `matrix-static-files`, but weren't serving it yet.

This patch makes sure it's served on the `matrix.` domain, which allows
people to set up serving on the base domain via a redirect or reverse-proxying.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3562