initial docu for airfoils and aixml/endnode

docs/documentation/libraries/airfoils/index.md

.../documentation/libraries/aixml/endnode/getting-started.md

docs/documentation/libraries/aixml/endnode/index.md

docs/documentation/libraries/aixml/endnode/valid_units.md

docs/documentation/libraries/aixml/index.md

docs/documentation/libraries/index.md