Documentation fixes (linting and broken links) (#5563)

# Lint sphinx documentation and fix broken links

This PR lints multiple warnings shown in generation of the project
documentation (using "make docs_linkcheck" and "make docs_build").
Additionally documentation internal links to (now?) non-existent files
are modified to point to existing documents as it seemed the new correct
target.

The documentation is not updated content wise.
There are no source code changes.

Fixes # (issue)

- broken documentation links to other files within the project
- sphinx formatting (linting)

## Before submitting

No source code changes, so no new tests added.

---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

docs/modules/chains/generic/async_chain.ipynb

@@ -9,7 +9,7 @@
     "\n",
     "LangChain provides async support for Chains by leveraging the [asyncio](https://docs.python.org/3/library/asyncio.html) library.\n",
     "\n",
-    "Async methods are currently supported in `LLMChain` (through `arun`, `apredict`, `acall`) and `LLMMathChain` (through `arun` and `acall`), `ChatVectorDBChain`, and [QA chains](../indexes/chain_examples/question_answering.html). Async support for other chains is on the roadmap."
+    "Async methods are currently supported in `LLMChain` (through `arun`, `apredict`, `acall`) and `LLMMathChain` (through `arun` and `acall`), `ChatVectorDBChain`, and [QA chains](../index_examples/question_answering.ipynb). Async support for other chains is on the roadmap."
    ]
   },
   {
@@ -104,7 +104,7 @@
     "s = time.perf_counter()\n",
     "generate_serially()\n",
     "elapsed = time.perf_counter() - s\n",
-    "print('\\033[1m' + f\"Serial executed in {elapsed:0.2f} seconds.\" + '\\033[0m')"
+    "print('\\033[1m' + f\"Serial executed in {elapsed:0.2f} seconds.\" + '\\033[0m')\n"
    ]
   }
  ],