Add Rhai book.

doc/src/start/builds/performance.md

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+Performance Build
+=================
+
+{{#include ../../links.md}}
+
+Use Only One Integer Type
+------------------------
+
+Some features are for performance.  For example, using [`only_i32`] or [`only_i64`] disables all other integer types (such as `u16`).
+If only a single integer type is needed in scripts - most of the time this is the case - it is best to avoid registering
+lots of functions related to other integer types that will never be used.  As a result, performance should improve.
+
+
+Use Only 32-Bit Numbers
+----------------------
+
+If only 32-bit integers are needed - again, most of the time this is the case - using [`only_i32`] disables also `i64`.
+On 64-bit targets this may not gain much, but on some 32-bit targets this improves performance due to 64-bit arithmetic
+requiring more CPU cycles to complete.
+
+
+Minimize Size of [`Dynamic`]
+---------------------------
+
+Turning on [`no_float`], and [`only_i32`] makes the key [`Dynamic`] data type only 8 bytes small on 32-bit targets
+while normally it can be up to 16 bytes (e.g. on x86/x64 CPU's) in order to hold an `i64` or `f64`.
+Making [`Dynamic`] small helps performance due to better cache efficiency.