Add section before the fast types to explain what they are

Author: mborland

doc/decimal.adoc

@@ -18,10 +18,11 @@ https://www.boost.org/LICENSE_1_0.txt
 include::decimal/overview.adoc[]
 include::decimal/generic_decimal.adoc[]
 include::decimal/decimal32.adoc[]
-include::decimal/decimal32_fast.adoc[]
 include::decimal/decimal64.adoc[]
-include::decimal/decimal64_fast.adoc[]
 include::decimal/decimal128.adoc[]
+include::decimal/fast_types.adoc[]
+include::decimal/decimal32_fast.adoc[]
+include::decimal/decimal64_fast.adoc[]
 include::decimal/decimal128_fast.adoc[]
 include::decimal/conversions.adoc[]
 include::decimal/literals.adoc[]

doc/decimal/fast_types.adoc

@@ -0,0 +1,14 @@
+////
+Copyright 2024 Matt Borland
+Distributed under the Boost Software License, Version 1.0.
+https://www.boost.org/LICENSE_1_0.txt
+////
+
+[#fast_types]
+= Fast Types
+:idprefix: fast_types_
+
+Now that we have seen the three basic types as specified in IEEE-754 there are three additional adjacent types: `decimal32_fast`, `decimal64_fast`, and `decimal128_fast`.
+These types yield identical computational results, but with ~3x faster performance.
+These types make the classic tradeoff of space for time as they require more storage width than the IEEE 754 conformant type.
+