[DOC] Add documentation of client certificate (mutual TLS) authentication for ML connectors

[ishag4]

What do you want to do?
Add documentation of client certificate authentication (mutual TLS) support for ML connectors.

• 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.
Connectors can now authenticate to remote endpoints with a client certificate (mutual TLS), in addition to the existing credential-based methods.

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
3.9

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

• Feature request: [FEATURE] Add Client Certificate Authentication Support for ML Connectors ml-commons#4515
• Implementation PR: Issue #4515: Add Client Certificate Authentication Support for ML Connectors ml-commons#4731

[ishag4]

Please assign it to me, thanks!