[Relay][Docs] Relay Language Reference (#2232)

docs/langref/index.rst

 Language Reference
 ==================
-This document provide references to
-embedded languages in TVM stack.
+This document provides references to
+embedded languages and IRs in the TVM stack.
+
+Introduction to Relay
+---------------------
+
+Relay is a functional, differentiable programming language
+designed to be an expressive intermediate representation for machine
+learning systems. Relay supports closures, control flow, and
+recursion, allowing it to directly represent more complex models
+than can computation graph-based IRs.
+Relay also includes a form of dependent typing using *type relations*
+in order to handle shape analysis for operators with complex
+requirements on argument shapes.
+
+Relay is extensible by design and makes it easy for machine learning
+researchers and practitioners to develop new large-scale program
+transformations and optimizations.
+
+The below pages describe the grammar, type system, and operators in Relay, respectively.
 
 .. toctree::
    :maxdepth: 2
 
+   relay_expr
+   relay_type
    relay_op
+
+Hybrid Script
+-------------
+
+The below page describes the TVM hybrid script front-end,
+which uses software emulation to support some constructs not
+officially supported in TVM.
+
+.. toctree::
+   :maxdepth: 2
+
    hybrid_script