docs: update main site layout (#2191)

* Remove land hand sidebar on most pages
* Cleans up some headings
* Adds error reference information to index (it was already on the
sidebar for the how-to page) -- should probably be its own tab?
* Adds an index page for the reference (so it's easier to link to a main
reference page), alternatively we can set up a redirect from index to
graph

docs/docs/reference/index.md

@@ -0,0 +1,10 @@
+---
+title: Reference
+description: API reference for LangGraph
+---
+
+# Reference
+
+Welcome to the LangGraph API reference! This reference provides detailed information about the LangGraph API, including classes, methods, and other components.
+
+If you are new to LangGraph, we recommend starting with the [Quick Start](../tutorials/introduction.ipynb) in the Tutorials section.