Move infra-setup and k3s-setup into dedicated directories

This changes makes it easier to differentiate and understand the
different parts of the kubernetes setup. On one hand we have the bare
infrastructure (servers, network, etc), on the other hand we have the
software (k3s in this case).

In the future we'll have a few more parts, like the minimal
configuration of the kubernetes cluster, e.g. with a cert-manager. This
is easier to manage with helm or terraform than with ansible. Therefore
it makes even more sense to split the responsibilities into dedicated
directories.

infra/README.adoc

@@ -0,0 +1,21 @@
+= infra
+:icons: font
+
+This project is responsible for providing the required infra to run a kubernetes-cluster.
+
+== Setup
+
+// tag::setup[]
+[WARNING]
+Make sure `config.auto.tfvars` with all the needed configuration-secrets is present otherwise the module cannot be applied!
+The file is savely stored in the password-manager.
+
+[source,bash]
+----
+terraform init # <1>
+terraform apply # <2>
+----
+
+<1> Init the terraform modules if necessary
+<2> Setup infrastructure and create/update inventory.ini (This might take some time, even after it's "ready")
+// end::setup[]