Rename "forensic" to "failure" in docs and dashboard configs

Update documentation files (output.md, usage.md, kibana.md, splunk.md,
elasticsearch.md, index.md, example.ini) and dashboard configurations
(Grafana JSON, Kibana ndjson, Splunk XML) to use "failure" terminology
instead of "forensic", consistent with the codebase rename.

- CLI args: --forensic-* → --failure-*
- Config keys: save_forensic → save_failure, forensic_topic → failure_topic, etc.
- Index names: dmarc_forensic → dmarc_failure
- Splunk dashboard: renamed file from dmarc_forensic_dashboard.xml to dmarc_failure_dashboard.xml
- Backward-compat note preserved: "formerly known as forensic reports"

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/source/example.ini

@@ -2,7 +2,7 @@
 
 [general]
 save_aggregate = True
-save_forensic = True
+save_failure = True
 
 [imap]
 host = imap.example.com