Changes
→Examples: Use source="yes" to replace (out-of-date) manual <texcode> block.
as first value after the <code>\dimexpr</code> command and then the numeric values.
Notice that the following expression is also valid:
<contextsource="yes">
\setupexternalfigures[location={local,default}]
\placesidebyside{\externalfigure[cow][width=.4\textwidth]}