docs: add an unstructured section to the ecosystem page (#1125)

### Summary

Adds an Unstructured section to the ecosystem page.

docs/ecosystem/unstructured.md

@@ -0,0 +1,45 @@
+# Unstructured
+
+This page covers how to use the [`unstructured`](https://github.com/Unstructured-IO/unstructured)
+ecosystem within LangChain. The `unstructured` package from
+[Unstructured.IO](https://www.unstructured.io/) extracts clean text from raw source documents like
+PDFs and Word documents.
+
+
+This page is broken into two parts: installation and setup, and then references to specific
+`unstructured` wrappers.
+
+## Installation and Setup
+- Install the Python SDK with `pip install unstructured[local-inference]`
+- Install the following system dependencies if they are not already available on your system.
+  Depending on what document types you're parsing, you may not need all of these.
+    - `libmagic-dev`
+    - `poppler-utils`
+    - `tesseract-ocr`
+    - `libreoffice`
+- Run the following to install NLTK dependencies. `unstructured` will handle this automatically
+  soon.
+  - `python -c "import nltk; nltk.download('punkt')"`
+  - `python -c "import nltk; nltk.download('averaged_perceptron_tagger')"`
+- If you are parsing PDFs, run the following to install the `detectron2` model, which
+  `unstructured` uses for layout detection:
+    - `pip install "detectron2@git+https://github.com/facebookresearch/detectron2.git@v0.6#egg=detectron2"`
+
+## Wrappers
+
+### Data Loaders
+
+The primary `unstructured` wrappers within `langchain` are data loaders. The following
+shows how to use the most basic unstructured data loader. There are other file-specific
+data loaders available in the `langchain.document_loaders` module.
+
+```python
+from langchain.document_loaders import UnstructuredFileLoader
+
+loader = UnstructuredFileLoader("state_of_the_union.txt")
+loader.load()
+```
+
+If you instantiate the loader with `UnstructuredFileLoader(mode="elements")`, the loader
+will track additional metadata like the page number and text type (i.e. title, narrative text)
+when that information is available.