docs: langchain-openai (#15513)

Updates docs and cookbooks to import ChatOpenAI, OpenAI, and OpenAI
Embeddings from `langchain_openai`

There are likely more

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

cookbook/hypothetical_document_embeddings.ipynb

@@ -22,8 +22,7 @@
    "source": [
     "from langchain.chains import HypotheticalDocumentEmbedder, LLMChain\n",
     "from langchain.prompts import PromptTemplate\n",
-    "from langchain_community.embeddings import OpenAIEmbeddings\n",
-    "from langchain_community.llms import OpenAI"
+    "from langchain_openai import OpenAI, OpenAIEmbeddings"
    ]
   },
   {