[devdoc] document sources in target platform zip & use of PDE Source Lookup feature

So that dev and qe can use Fred's PDE source lookup tool [1] to grab source bundles on demand (instead of having to build a target platform locally w/ all the sources [2]), we need to build a targetplatform with all the sources - done in JBIDE-23481.

[1] https://github.com/fbricon/pde.source.lookup/
[2] https://github.com/jbosstools/jbosstools-devdoc/blob/master/building/target_platforms/target_platforms_for_consumers.adoc#build-target-platform-bootstrapped-from-zip

Once we have sources in the JBT targetplatform repo, need to update devdoc [2] and add instructions (also write a blog!) about how to install/use the PDE Source Lookup plugin

1. install the PDE source lookup plugin
2. Window > Prefs > Update > Avail Software Sites > Add the new tp.source site
3. for something you want sources, use the plugin and see if it can resolve sources