reorganize module

_archive/rhai_engine/rhaibook/ref/modules/index.md

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+Modules
+=======
+
+Rhai allows organizing code into _modules_.
+
+A module holds a collection of [functions](../functions.md), [constants](../constants.md) and sub-modules.
+
+It may encapsulates a Rhai script together with the [functions](../functions.md) and
+[constants](../constants.md) defined by that script.
+
+Other scripts can then load this module and use the [functions](../functions.md) and
+[constants](../constants.md) exported as if they were defined inside the same script.