doc: break the constraint of test images

[ttzeng]

In the current Makefile, names of level-1 subdirectories under the demo folder are collected and set as make targets for building the test images, regardless whether the subdirectories contain docker build files. This implication together with the associated yaml files and the configuration files leads to a very flat demo directory:

demos = $(shell basename -a demo/*/)

$(demos):
	@cd demo/ && ./build-image.sh $(REG)$@ $(BUILDER)

To organize the demo content by device types, we need to break this constraint but still keep the valid make targets (i.e. make demos and make <test-image-name>)