Reference beforeSend in service guide and improve highlighting

[justinbmeyer]

https://canjs.com/doc/can-connect/data/url/url.url.html

The examples are not highlighted:

They also are not codepen-able.

Finally, we should link to beforeSend in the guide around here: https://canjs.com/doc/guides/data-configuring-requests.html#CustomizingtherequestmethodandURL

[nlundquist]

Added link to beforeSend in guide:
canjs/canjs#5311
#502

[nlundquist]

Added codepen examples to #502

[phillipskevin]

so can this be closed @nlundquist? or is there more to do?