#4998 Doc: CG - update information for compiling LP with regard to new URW++ fonts

This came out of issue 4991.

We need to update the CG to include information about the URW++ fonts that are now possible to use when compiling LP snippets and where configure will post a warning for those users that do not have these fonts installed.

Currently the fonts we need are a tiny subset of a much larger font family(families?) so instructions to install just these few fonts as described in https://codereview.appspot.com/315850043/ is, at the moment, not so simple.

There is a discussion about either documenting how to 'wget' the specific commits from the URW++ git repo or perhaps tar the files up so developers can download them from the LP website. That is still TBD.