docs: Consolidate "Design documents" files

Documentation/Doxygen/DataAssembly.md

Documentation/Doxygen/IOLegacyInformationFormat.md

Documentation/Doxygen/IOXMLInformationFormat.md

Documentation/Doxygen/IOXMLTimeInFieldData.md

Documentation/Doxygen/SMPTools.md

...tation/docs/design_documents/IOLegacyInformationFormat.md

...mentation/docs/design_documents/IOXMLInformationFormat.md

Documentation/docs/design_documents/IOXMLTimeInFieldData.md

Documentation/docs/design_documents/data_assemply.md

Documentation/docs/design_documents/smptools.md