docs: Update callbacks documentation (#18899)

**Description:** Update callbacks documentation
**Issue:** Change some module imports and a method invocation to reflect
the current LangChainAPI
**Dependencies:** None

docs/docs/modules/callbacks/custom_callbacks.ipynb

@@ -52,7 +52,7 @@
     }
    ],
    "source": [
-    "from langchain.callbacks.base import BaseCallbackHandler\n",
+    "from langchain_core.callbacks import BaseCallbackHandler\n",
     "from langchain_core.messages import HumanMessage\n",
     "from langchain_openai import ChatOpenAI\n",
     "\n",
@@ -66,7 +66,7 @@
     "# Additionally, we pass in a list with our custom handler\n",
     "chat = ChatOpenAI(max_tokens=25, streaming=True, callbacks=[MyCustomHandler()])\n",
     "\n",
-    "chat([HumanMessage(content=\"Tell me a joke\")])"
+    "chat.invoke([HumanMessage(content=\"Tell me a joke\")])"
    ]
   },
   {

docs/docs/modules/callbacks/index.mdx

@@ -79,15 +79,15 @@ class BaseCallbackHandler:
 
 ## Get started
 
-LangChain provides a few built-in handlers that you can use to get started. These are available in the `langchain/callbacks` module. The most basic handler is the `StdOutCallbackHandler`, which simply logs all events to `stdout`.
+LangChain provides a few built-in handlers that you can use to get started. These are available in the `langchain_core/callbacks` module. The most basic handler is the `StdOutCallbackHandler`, which simply logs all events to `stdout`.
 
 **Note**: when the `verbose` flag on the object is set to true, the `StdOutCallbackHandler` will be invoked even without being explicitly passed in.
 
 ```python
-from langchain.callbacks import StdOutCallbackHandler
+from langchain_core.callbacks import StdOutCallbackHandler
 from langchain.chains import LLMChain
 from langchain_openai import OpenAI
-from langchain.prompts import PromptTemplate
+from langchain_core.prompts import PromptTemplate
 
 handler = StdOutCallbackHandler()
 llm = OpenAI()
@@ -160,5 +160,5 @@ The `verbose` argument is available on most objects throughout the API (Chains,
 ### When do you want to use each of these?
 
 - Constructor callbacks are most useful for use cases such as logging, monitoring, etc., which are _not specific to a single request_, but rather to the entire chain. For example, if you want to log all the requests made to an `LLMChain`, you would pass a handler to the constructor.
-- Request callbacks are most useful for use cases such as streaming, where you want to stream the output of a single request to a specific websocket connection, or other similar use cases. For example, if you want to stream the output of a single request to a websocket, you would pass a handler to the `call()` method
+- Request callbacks are most useful for use cases such as streaming, where you want to stream the output of a single request to a specific websocket connection, or other similar use cases. For example, if you want to stream the output of a single request to a websocket, you would pass a handler to the `invoke()` method