XML Interface inheritance documentation
XML interface inheritance is hardly documented in our wiki, doxygen and .md files.
It is used without explanation in the plugin wiki page. https://www.paraview.org/Wiki/ParaView/Plugin_HowTo
It is mentioned without explanation in the properties panel wiki page. https://www.paraview.org/Wiki/ParaView/Properties_Panel
vtkSMProxy doxygen explain what it is used for without giving an example. https://kitware.github.io/paraview-docs/latest/cxx/classvtkSMProxy.html
I would suggest adding a .md file, in the same idea of ProxyHints.md and also referencing the new example plugin in !2970 (merged)
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