[Relay][Docs] Documentation for Algebraic Data Types (#2575)

Author: slyubomirsky

docs/langref/index.rst

@@ -8,9 +8,9 @@ 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.
+learning systems. Relay supports algebraic data types, closures,
+control flow, and recursion, allowing it to directly represent more
+complex models than computation graph-based IRs can.
 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.
@@ -19,13 +19,15 @@ 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.
+The below pages describe the grammar, type system,
+algebraic data types, and operators in Relay, respectively.
 
 .. toctree::
    :maxdepth: 2
 
    relay_expr
    relay_type
+   relay_adt
    relay_op
 
 Hybrid Script