Docs refactor (#480)

Big docs refactor! Motivation is to make it easier for people to find
resources they are looking for. To accomplish this, there are now three
main sections:

- Getting Started: steps for getting started, walking through most core
functionality
- Modules: these are different modules of functionality that langchain
provides. Each part here has a "getting started", "how to", "key
concepts" and "reference" section (except in a few select cases where it
didnt easily fit).
- Use Cases: this is to separate use cases (like summarization, question
answering, evaluation, etc) from the modules, and provide a different
entry point to the code base.

There is also a full reference section, as well as extra resources
(glossary, gallery, etc)

Co-authored-by: Shreya Rajpal <ShreyaR@users.noreply.github.com>

docs/reference/installation.md

@@ -1,4 +1,6 @@
-# Installation Options
+# Installation
+
+## Official Releases
 
 LangChain is available on PyPi, so to it is easily installable with:
 
@@ -27,4 +29,12 @@ Note that if you are using `zsh`, you'll need to quote square brackets when pass
 
 ```
 pip install 'langchain[all]'
-```
+```
+
+## Installing from source
+
+If you want to install from source, you can do so by cloning the repo and running:
+
+```
+pip install -e .
+```