Impl. createDLNode(), insertHead(), testDLList().

data_structures/doubly_linked_list/src/doubly_linked_list.(c, h)
	-Implement and document `createDLNode()` and `insertHead()`.

data_structures/doubly_linked_list/src/test.c
	-Implement and document `testDLList()`, and move all test
	statements from `main()` to it for greater modularity.
	-Redefine `freeData()` as static, because it was unnecessarily
	exposed to the global scope.