Add a documentation chapter about the category theory module.

The page currently doesn't currently do much beyond pulling in the
corresponding docstrings and, in fact, that's the purpose of adding the
chapter at this moment.

doc/src/modules/categories.txt

@@ -0,0 +1,27 @@
+Category Theory Module
+======================
+
+.. module:: sympy.categories
+
+Introduction
+------------
+
+The category theory module for SymPy will allow manipulating diagrams
+within a single category, including drawing them in TikZ and deciding
+whether they are commutative or not.
+
+The module is yet in its pre-embryonic stage.
+
+Base Class Reference
+--------------------
+.. autoclass:: Object
+   :members:
+
+.. autoclass:: Morphism
+   :members:
+
+.. autoclass:: Category
+   :members:
+
+.. autoclass:: Diagram
+   :members:

doc/src/modules/index.txt

@@ -47,6 +47,7 @@ access any SymPy module, or use this contens:
    utilities/index.txt
    parsing.txt
    physics/index.txt
+   categories.txt
 
 Contributions to docs
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