[DOC] Correct document for dashboards assistant

[yuye-aws]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

Some part document of the dashboard assistant is not correct in this PR #8622. To be specific, the API in the following two files should not be POST /_plugins/_ml/agents/_register, but POST /_plugins/_flow_framework/workflow instead.

1. _dashboards/dashboards-assistant/suggest-anomaly-detector.md
2. _dashboards/dashboards-assistant/data-summary.md

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.18

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[yuye-aws]

Also, the following request mentioned here is directly writing into the system index .plugins-ml-config. This system index cannot be updated when security plugin is enabled.

POST /.plugins-ml-config/_doc/os_insight
{
  "type": "os_root_agent",
  "configuration": {
    "agent_id": "<ALERT_INSIGHTS_AGENT_ID>"
  }
}

[yuye-aws]

@kolchfa-aws Can I get assigned to this issue? I'll create a PR to update the current doc.