PublicShow -- Serve example files

Locate and serve files for the Examples menu as well as examples included from overview notebooks. The examples come from two sources:

This module also makes the known examples available through swish_provides/1 for supporting conditional statements on example overview notebooks.

Source md_eval:provides(?Term) is nondet[multifile]
Make examples available through swish_provides/1. Can be used in dynamic cells as, e.g.,:
:- if(swish_provides(example('',_,_))).
:- endif.