Add platform support to create an Applied To section

[EWSoftware]

Add a "platform builder" modeled after the version builder plug-in but integrated into the build engine. It would evaluate all project documentation sources, group them by target framework, and build reflection data for each set. If a project multi-targets, it would be included once for each target framework. The separate reflection info files would be merged similar to how version builder works but would add platform elements that specify which platforms and versions each API member applies to. This info could then be used to render an Applied To section similar to Microsoft Docs. This would also solve the current problem of not being able to document assemblies from mixed sets of incompatible target frameworks.

The Framework Version property in the project would effectively only be used for non-project documentation sources. The version builder plug-in would be used to render a version section that applies to different versions of the projects being documented. It still needs some work though to make it more usable.