[Documentation]: Fix example playbook YAML and document Beats output defaults #520

The README example playbook has issues and the Beats output defaults are undocumented.

• YAML indentation error in the rsyslog pre_task (ansible.builtin.service indented 7 spaces instead of 6) → copy-paste fails.
• The example silently assumes Logstash: beats_filebeat_output defaults to logstash, so without a Logstash node Filebeat ships nowhere. Add a note — with Logstash set the Beats outputs to logstash, without it to elasticsearch.
• Document the current output defaults (filebeat → logstash, auditbeat/metricbeat → elasticsearch) and note they are inconsistent (unification tracked in [Feature]: Unify default Beats output to elasticsearch #519).
• Correct the stated elasticstack_release default 7 → 8 (README.md:103, docs/getting-started.md:62; check meta/argument_specs.yml too).

Docs only, non-breaking. Related: #422.