Clarify Default Chat API Key creation for existing instances

- Specify that Default Chat API Key is only auto-created for fresh instances
- Add note for users with existing instances
- Include curl example for manually creating a chat API key
- Document chatCompletions action in API keys reference

This addresses the issue where users with existing Meilisearch instances
need to manually create their chat API key since the default one is only
created for fresh instances.

Author: tpayet

learn/chat/getting_started_with_chat.mdx

@@ -30,14 +30,40 @@ curl \
 
 ## Find your chat API key
 
-When Meilisearch runs with a master key, it automatically creates a "Default Chat API Key" with `chatCompletions` permission. Find it using:
+When Meilisearch runs with a master key on a **fresh instance**, it automatically creates a "Default Chat API Key" with `chatCompletions` permission.
+
+<Note>
+The Default Chat API Key is only created automatically for new Meilisearch instances. If you have an existing instance, you'll need to create your own API key with `chatCompletions` permission.
+</Note>
+
+Find existing keys using:
 
 ```bash
 curl http://localhost:7700/keys \
   -H "Authorization: Bearer MEILISEARCH_KEY"
 ```
 
-Look for the key with "Default Chat API Key" in the description. Use this key when queryin the `/chats` endpoint.
+Look for the key with "Default Chat API Key" in the description. Use this key when querying the `/chats` endpoint.
+
+### Creating a chat API key for existing instances
+
+If you have an existing Meilisearch instance and don't see a Default Chat API Key, create one manually:
+
+```bash
+curl \
+  -X POST 'http://localhost:7700/keys' \
+  -H 'Authorization: Bearer MEILISEARCH_KEY' \
+  -H 'Content-Type: application/json' \
+  --data-binary '{
+    "name": "Chat API Key",
+    "description": "API key for chat completions",
+    "actions": ["search", "chatCompletions"],
+    "indexes": ["*"],
+    "expiresAt": null
+  }'
+```
+
+This creates an API key with both `search` and `chatCompletions` permissions, which are required for the chat feature.
 
 ## Configure your indexes for chat
 

reference/api/keys.mdx

@@ -110,6 +110,7 @@ For security reasons, we do not recommend creating keys that can perform all act
 | **`keys.delete`**      | Provides access to the [delete key](#delete-a-key) endpoint                                                                                                                                                                                                                                                                  |
 | **`network.get`**      | Provides access to the [get the network object](/reference/api/network#get-the-network-object) endpoint                                                                                                                                                                                                                                                                |
 | **`network.update`**   | Provides access to the [update the network object](/reference/api/network#update-the-network-object) endpoint                                                                                                                                                                                                                                                                  |
+| **`chatCompletions`**  | Provides access to the [chat completions endpoints](/reference/api/chats). Requires experimental feature to be enabled. Usually paired with `search` action                                                                                                                                                                                                                     |
 
 ### `indexes`