CIF language tutorial: change 'Extensions' section into a 'Synthesis' section

After #1297 (closed), the 'Extensions' section only contains the 'Supervisory controller synthesis' lesson. I propose to change the 'Extensions' section into a 'Supervisory controller synthesis' section, and split the 'Supervisory controller synthesis' lesson into multiple lessons, for plant/requirement/supervisor, controllable/uncontrollable, marking, etc. It is one of the largest lessons we have (looking at the file size of all AsciiDoc files in the language tutorial folders, recursively). Also, the sub-topics are really separate concepts, that could better be explained in sub-sections.