Use MSBUILD to configure DocFX / Better MSBUILD integration

It would be nice to integrate DocFX better into the MSBUILD ecosystem.

That would make align DocFX with .NET Core, and make it easier to configure DocFX in big solutions, use dynamic build metadata, extend DocFX via nuget packages...

- use msbuild properties to configure docfx properties, like docfx.json->build.dest, docfx.metadata.json
- use msbuild item groups to configure templates, preprocessors, content, resources...
- use for referencing projects for which to generate API documentation
- use for extending DocFX with templates, preprocessors...
- use for referencing projects for which to generate API documentation
- create DocFX project type. Create msbuild [SDK](https://github.com/Microsoft/MSBuildSdks)
- use msbuild targets to configure the docfx build/tooling pipeline, which would also make it easier to extend phases.
- Support the "standard" build log message format, so that warnings and errors are displayed correctly in IDE, and goto works
- Support building multiple output formats