Created a glossary and added one new entry (GH-3810)

docs/src/userguide/glossary.rst

+.. glossary::
+
+   pointer
+      A **pointer** is a variable that stores the address of another variable (i.e. direct address of the memory location). They allow for dynamic memory allocation and deallocation. They can be used to build dynamic data structures. `Read more <https://en.wikipedia.org/wiki/Pointer_(computer_programming)#C_pointers>`__.

docs/src/userguide/index.rst

@@ -35,3 +35,4 @@ Indices and tables
 
 .. toctree::
 
+   glossary