[doc] improve documentation of make targets and ./manage script

BTW force modularization of the ./mange script into sub modules:

- utils/lib_sxng_data.sh
- utils/lib_sxng_node.sh
- utils/lib_sxng_static.sh
- utils/lib_sxng_test.sh
- utils/lib_sxng_themes.sh

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>