Move script optimization into separate section.

doc/src/rust/modules/index.md

@@ -0,0 +1,14 @@
+Modules
+=======
+
+{{#include ../../links.md}}
+
+Rhai allows organizing code (functions, both Rust-based or script-based, and variables) into _modules_.
+Modules can be disabled via the [`no_module`] feature.
+
+A module is of the type `Module` and holds a collection of functions, variables, iterators and sub-modules.
+It may be created entirely from Rust functions, or it may encapsulate a Rhai script together with the functions
+and variables defined by that script.
+
+Other scripts can then load this module and use the functions and variables exported
+as if they were defined inside the same script.