Remove redundant .docx loader (closes #1716) + update how_to_guides.rst (#1891)

In https://github.com/hwchase17/langchain/issues/1716 , it was
identified that there were two .py files performing similar tasks. As a
resolution, one of the files has been removed, as its purpose had
already been fulfilled by the other file. Additionally, the init has
been updated accordingly.

Furthermore, the how_to_guides.rst file has been updated to include
links to documentation that was previously missing. This was deemed
necessary as the existing list on
https://langchain.readthedocs.io/en/latest/modules/document_loaders/how_to_guides.html
was incomplete, causing confusion for users who rely on the full list of
documentation on the left sidebar of the website.

docs/modules/document_loaders/examples/word_document.ipynb

@@ -27,7 +27,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "loader = UnstructuredWordDocumentLoader(\"fake.docx\")"
+    "loader = UnstructuredWordDocumentLoader(\"example_data/fake.docx\")"
    ]
   },
   {
@@ -78,7 +78,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "loader = UnstructuredWordDocumentLoader(\"fake.docx\", mode=\"elements\")"
+    "loader = UnstructuredWordDocumentLoader(\"example_data/fake.docx\", mode=\"elements\")"
    ]
   },
   {