[DOC] Update dynamic CSP rule configuration to only allow frame-ancestors

[tianleh]

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 and all versions that are affected.
This is about an existing document. https://github.com/opensearch-project/documentation-website/blob/main/_dashboards/csp/csp-dynamic-configuration.md

In the existing doc for 2.13.0, we allowed customers to configure the whole CSP rule. To avoid customers accidentally change such, we have made a change to limit customers to configure only one directive frame-ancestors of the CSP rule. This change will go to 2.14.0 as well as future patch releases for 2.13.

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

The README.md file has been updated to reflect the current situation.
https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/src/plugins/csp_handler/README.md

[tianleh]

I am creating a PR for such.