[ML] Add an AI settings section into Stack Management

[peteharverson]

Create a dedicated 'AI' section inside Stack Management, with a 'GenAI settings' page, and moving in the existing AI Assistants page from its current location inside the 'Kibana' section.

The new GenAI settings page will initially contain sections for:

• AI Connector - with a link to the Connectors page to manage AI connectors (and possibly a link to create a new AI Connector by opening the 'Create AI Connector' flyout).
• AI feature visibility (not shown on Serverless or when a space has a solution UI picked) - with a link to the Spaces page to enable or disable AI-powered features.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[elasticmachine]

Pinging @elastic/appex-ai-infra (Team:AI Infra)