[DOC] Add cross-encoder model

[ylwu-amzn]

What do you want to do?

We will release a new model type in 2.12 opensearch-project/ml-commons#1615. Need to update doc for adding this model.

• 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.

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