Doc: introduce main documentation for Scripting API

doc/scripting/mainpage.md

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+# Introduction to Mayo Scripting
+
+Mayo application can be scripted with Javascript language. Mayo provides a scripting API designed to
+access the application data and some features. This includes:
+- Traversal of model tree: visit assemblies, instances, parts, ...
+- Access to shapes: get the [topologic shape](https://dev.opencascade.org/doc/overview/html/occt_user_guides__modeling_data.html#occt_modat_5) at any model tree node, 
+- Access to geometries: get the geometry support from any topologic face(surface), edge(curve) or vertex(point)
+- ...