Add the grammar to the reference manual, since the new docs don't

have the feature of putting all the small EBNF snippets together
into one big file.

Doc/reference/grammar.rst

+Full Grammar specification
+==========================
+
+This is the full Python grammar, as it is read by the parser generator and used
+to parse Python source files:
+
+.. literalinclude:: ../../Grammar/Grammar

Doc/reference/index.rst

@@ -27,4 +27,4 @@ interfaces available to C/C++ programmers in detail.
    simple_stmts.rst
    compound_stmts.rst
    toplevel_components.rst
-
+   grammar.rst