Various old guides mention this, but it seems like neither Element Web,
nor Element Desktop make use of it.
Element Web & Element Desktop use their own `config.json` configuration to figure out where
Element Call is.
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
This is a break in backward-compatibility for people disabling
`index.html` creation via the playbook but are managing their static
website files in another way (AUX role, etc).
