Fix: Updated links for in depth explanation of chain types in the Question Answering notebooks (#3714)

In the notebook question_answering.ipynb
([link](https://github.com/hwchase17/langchain/blob/master/docs/modules/chains/index_examples/question_answering.ipynb)),
and the notebook qa_with_sources.ipynb
([link](https://github.com/hwchase17/langchain/blob/master/docs/modules/chains/index_examples/qa_with_sources.ipynb)),
the first paragraph contains a dead link:

> This notebook walks through how to use LangChain for question
answering over a list of documents. It covers four different types of
chains: stuff, map_reduce, refine, map_rerank. For a more in depth
explanation of what these chain types are, see
[here](32793f94fd/docs/modules/chains/combine_docs.md).

The file combine_docs.md doesn't exist anymore and thus provide 404 -
Page not found.

I updated the links so it redirect to
https://docs.langchain.com/docs/components/chains/index_related_chains
as in the summarize notebook
([link](https://github.com/hwchase17/langchain/blob/master/docs/modules/chains/index_examples/summarize.ipynb))
present in the same folder.

docs/modules/chains/index_examples/qa_with_sources.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Question Answering with Sources\n",
     "\n",
-    "This notebook walks through how to use LangChain for question answering with sources over a list of documents. It covers four different chain types: `stuff`, `map_reduce`, `refine`,`map-rerank`. For a more in depth explanation of what these chain types are, see [here](../combine_docs.md)."
+    "This notebook walks through how to use LangChain for question answering with sources over a list of documents. It covers four different chain types: `stuff`, `map_reduce`, `refine`,`map-rerank`. For a more in depth explanation of what these chain types are, see [here](https://docs.langchain.com/docs/components/chains/index_related_chains)."
    ]
   },
   {

docs/modules/chains/index_examples/question_answering.ipynb

@@ -7,7 +7,7 @@
    "source": [
     "# Question Answering\n",
     "\n",
-    "This notebook walks through how to use LangChain for question answering over a list of documents. It covers four different types of chains: `stuff`, `map_reduce`, `refine`, `map_rerank`. For a more in depth explanation of what these chain types are, see [here](../combine_docs.md)."
+    "This notebook walks through how to use LangChain for question answering over a list of documents. It covers four different types of chains: `stuff`, `map_reduce`, `refine`, `map_rerank`. For a more in depth explanation of what these chain types are, see [here](https://docs.langchain.com/docs/components/chains/index_related_chains)."
    ]
   },
   {