Add some chapter introductions (not linked into the LibRef yet)
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+\chapter{Numeric and Mathematical Modules}
+\label{numeric}
+
+The modules described in this chapter provide
+numeric and math-related functions and data types.
+The \module{math} and \module{cmath} contain
+various mathematical functions for floating-point and complex numbers.
+For users more interested in decimal accuracy than in speed, the
+\module{decimal} module supports exact representations of decimal numbers.
+
+The following modules are documented in this chapter:
+
+\localmoduletable