Add support for a 'product' build target.
The doc infrastructure already supports 'pdf' and
'publicsite' targets; the latter is used to generate
the HTML hosted on perforce.com.
Newer products, such as P4Insights and Swarm 2.0,
will need to also produce HTML that will be hosted
by the product, and which should be styled (and
possibly interact) like the product itself.
This change includes a mechanism to cause the build
to end without failure if a guide does not provide
'product' assets, specifically it looks for:
<guide>/assets/product/build.properties
If that file exists, the 'product' build continues as
expected, otherwise it will report going through the
motions without actually running any tasks.
Currently, the 'product' HTML is identical to the
'publicsite' HTML. Guides that require product-specific
HTML are required to customize both XSLT and CSS
as necessary, but this gives them the standard
presentation and functionality to start with.