📝 Guidance for what an Sdk looks like

What makes a good Sdk? What functionality must it provide? Should it always include Common.targets? Should we define a new minimum target surface?

What do dependencies look like?

Must every Sdk have both props and targets? Should an Sdk define a version-number that can be used in other Sdks for error checking?

We should pull heavily from the experience of the existing Web and NET Sdks, but temper that with understanding that they were originally defined for something else.

Later edits:

- the following page contains samples of msbuild sdks, as well as links to documentation: https://github.com/Microsoft/MSBuildSdks