update para on Xavier's development technique

Signed-off-by: Fotis Georgatos <fotis@cern.ch>
address Ken remarks on Concepts_and_Terminology

Signed-off-by: Fotis Georgatos <fotis@cern.ch>
add Configuration_Legacy.rst, update index and bump minor

Signed-off-by: Fotis Georgatos <fotis@cern.ch>
address Ken remarks on Configuration.rst

Signed-off-by: Fotis Georgatos <fotis@cern.ch>
address Ken remarks on Installation.rst

Signed-off-by: Fotis Georgatos <fotis@cern.ch>