Sphinx docs for various sidre-conduit hierarchy copies

[gunney1]

This was requested during MR #1510, where a sidre-to-conduit deep copy was implemented. There are 4 methods that "resemble" deep-copy and shallow-copy, sidre-to-conduit and conduit-to-sidre. "Resemble" because importConduitTreeExternal is listed as the shallow conduit-to-sidre but doesn't do exactly that. Also because the names are not naturally descriptive of their copy function but rather a result of some historical use. We should

• provide sphinx documentation for the hierarchy copies between Sidre and Conduit.
• discuss renaming the methods to make easier to see the difference between them.
• discuss whether importConduitTreeExternal should be fixed to do the shallow copy in the standard sense, or whether to write a separate conduit-to-sidre shallow copy.