docs: add READMEs to roles powerdns_tsig_key and dns

roles/powerdns_tsig_key/README.md

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+# `finallycoffee.base.powerdns_tsig_key`
+
+Simple ansible role for ensuring a TSIG key is present in a given PowerDNS-
+instance.
+
+## Usage
+
+The usage example below assumes `powerdns` is running in a container named `powerdns` (as supplied to `powerdns_tsig_key_container_name`.
+
+```yaml
+- hosts: "{{ target_hosts }}"
+  become: true
+  roles:
+    - role: finallycoffee.base.powerdns_tsig_key
+      vars:
+        powerdns_tsig_key_name: "nameofmykey"
+        powerdns_tsig_key_path: "/var/lib/myapp/tsig.key"
+        powernds_tsig_key_algo: "hmac-sha512"
+        powerdns_tsig_key_path_owner: "myappuser"
+        powerdns_tsig_key_path_group: "myappgroup"
+        powerdns_tsig_key_container_name: 'powerdns'
+```
+
+> [!INFO]
+> Support for non-docker deployments is pending.