* The ingress assigns an SFT allocation request to a random SFT
* Each sftd pod is made aware of an URL on which it is directly
reachable, and will return the URL in the response to the client.  e.g.
Pod `sftd-0` will be assigned `https://sft.example.com/sfts/sftd-0`
* The client tells this URL to other clients willing to join the call
* Other clients make a request to this URL
* The ingress points requests to `/sfts` to the `join-call` deployment,
which will redirect to the specific pod, such that the client can join
the conference call of the other client

some kubernetes clusters (like Scaleway) don't name the DNS server
kube-dns but core-dns.  /etc/resolv.conf is guaranteed to point to the
correct thing though.

This makes it quicker to scale up and down. As sft is actually not
stateful and does not require ordered restarts. We're just using
StatefulSet to get a persistent DNS name so we can join existing calls.

Co-authored-by: Lucendio <gregor.jahn@wire.com>

We can make deployment of sftd a bit easier in the future. Added a
section on this so I do not forget :)

Also make the docs a lot nicer.