docs: explain why host_key_checking is disabled in ansible.cfg

Every other security trade-off in this repo has an inline rationale;
this one didn't. Documents why it's off and what it costs you.

Author: saidalba

ansible/ansible.cfg

@@ -1,5 +1,10 @@
 [defaults]
 inventory = inventory
+# Disabled because destroying and recreating an instance (make down; make up)
+# reissues a fresh host key on the same/a reused IP, which would otherwise
+# trip Ansible's "REMOTE HOST IDENTIFICATION HAS CHANGED" refusal every time.
+# Trade-off: this skips MITM detection via SSH host key pinning. Verify
+# public_ip out-of-band (e.g. your cloud console) if that matters to you.
 host_key_checking = False
 retry_files_enabled = False
 interpreter_python = auto_silent