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

roles/k8s-setup/tasks/main.yml

@@ -1,17 +1,20 @@
 - name: Ensure cert-manager
   tags:
+  - init
   - k8s
   - cert-manager
   import_tasks: _cert-manager.yml
 
 - name: Ensure gitea
   tags:
+  - init
   - k8s
   - gitea
   import_tasks: _gitea.yml
 
 - name: Ensure concourse
   tags:
+  - init
   - k8s
   - concourse
   import_tasks: _concourse.yml