docs: add output_modules to preset YAML examples

[liquidsec]

Summary

• Preset YAML examples for output modules (discord, slack, teams, webhook, elastic, splunk, postgres, mysql) were missing output_modules:, so users who copied them would get silent failure
• Added output_modules: to every preset example and clarified the intro text

Fixes #1285

See also #3146 for a related issue with the output module override behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90%. Comparing base (db3e012) to head (a901c0c).
⚠️ Report is 11 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #3147   +/-   ##
=====================================
- Coverage     90%     90%   -0%     
=====================================
  Files        441     441           
  Lines      39003   39003           
=====================================
- Hits       34915   34911    -4     
- Misses      4088    4092    +4     

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