doc: Adding new architecture document describing the data model

doc/source/arch_data_model.rst

doc/source/image-sources/arch-datamodel-context.odg

...urce/image-sources/arch-datamodel-element-composition.odg

doc/source/image-sources/arch-datamodel-element.odg

...ource/image-sources/arch-datamodel-source-composition.odg

doc/source/image-sources/arch-datamodel-source.odg

doc/source/images/arch-datamodel-context.svg

doc/source/images/arch-datamodel-element-composition.svg

doc/source/images/arch-datamodel-element.svg

doc/source/images/arch-datamodel-source-composition.svg

doc/source/images/arch-datamodel-source.svg

doc/source/main_architecture.rst