docs: Restructure and improve AI configuration docs (#35133)

Adding a number of settings that weren't documented, restructuring
things a bit to separate what is model-related settings from agent panel
usage-related settings, adding the recently introduced `disable_ai` key,
and more.

Release Notes:

- Improved docs around configuring and using AI in Zed

docs/src/configuring-zed.md

@@ -3390,26 +3390,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
 
 ## Agent
 
-- Description: Customize agent behavior
-- Setting: `agent`
-- Default:
-
-```json
-"agent": {
-  "version": "2",
-  "enabled": true,
-  "button": true,
-  "dock": "right",
-  "default_width": 640,
-  "default_height": 320,
-  "default_view": "thread",
-  "default_model": {
-    "provider": "zed.dev",
-    "model": "claude-sonnet-4"
-  },
-  "single_file_review": true,
-}
-```
+Visit [the Configuration page](/ai/configuration.md) under the AI section to learn more about all the agent-related settings.
 
 ## Outline Panel