Top level toggle to switch from npm to yarn instructions

[mehulkar]

It can be confusing to constantly read references to yarn and npm. It would be much simpler for the reader to select npm or yarn once and read the whole set of guides in that context.

[jenweber]

This issue is still relevant. We will need to solve this for showing TypeScript examples in the docs, and that project is underway. This ticket should not be picked up until someone checks in on the status.

See also emberjs/rfcs#831