Document how to change the default from Elastic Managed LLM

[szabosteve]

Description

Make it clear how to change the default AI connector from the Elastic Managed LLM.
Customers can either configure another AI connector and switch to that as the default. They can do it in the Search or Security AI assistants by choosing another connector. Or they can disable all AI features through space settings if they don't want to use any AI connectors.

Impacted pages:

• Security AI assistant
• Elastic AI assistant for O11y and Search

Resources

Relevant Slack thread

[shubhaat]

I think this will be addressed with elastic/kibana#231940. Let's close this for now.