Enhance the README a lot

This change is huge, therefore I only sum up the most important changes:
* Improve spelling
* Reduce ambiguity
* Use OpenTofu instead of Terraform
* Document missing tags for Ansible
* Provide example-configuration
* Fix confusion between dotenv and direnv, I use direnv!
* Add section about required software
* Many spelling mistakes

config.auto.tfvars.tpl

@@ -0,0 +1,23 @@
+hetzner_dns_apitoken   = "YOUR_HETZNER_DNS_API_TOKEN"
+hetzner_cloud_apitoken = "YOUR_HETZNER_CLOUD_API_TOKEN"
+
+# Hetzner-locations: https://docs.hetzner.com/cloud/general/locations/
+k8s_servers = [
+  { type = "cax11", location = "fsn1", ip_datacenter = "fsn1-dc14" }
+]
+k8s_agents = [
+  { type = "cax11", location = "fsn1" }
+]
+
+ssh_keys = {
+  "SSH_KEY_NAME" = "YOUR_PUBLIC_SSH_KEY"
+}
+
+dns_zones = {
+  "example.com" = {
+    custom_records = [
+      { name = "@", ttl = 86400, type = "TXT", value = "HELLO" }
+    ]
+  }
+  "example.net" = {}
+}