placeholder: more flexible data placement using @embed instead of --path

[jpluscplusm]

This is a unpolished placeholder to track a general desire for pages explaining how to use @embed in place of --path, for more flexible data placement.

From Slack:

A cuelang.org page that shows an example of how embed can overcome shortcomings of -l.
The most obvious instance being want to place two data files at different locations