* add latex support for exercise admonition
* latex freezes/crashes on {HEAVY WIDE-HEADED RIGHTWARDS ARROW} so
replace them by more manual arrows
* add some preamble configuration, may not even be necessary
* generate WS setup code only in HTML output. WS doc in latex still
isn't great as it displays all 4 languages one after the other,
ideally they should be tagged or something, so only one language at a
time is generated in non-HTML outputs
Xavier Morel