Playground link tracks master

[Mirko-von-Leipzig]

The API playground is directly linked to whatever the spec is on master. This is an issue as users will use the release tag and presumably use the playground link to explore the API, as the raw spec text is difficult to navigate.

However, the link is always to a single playground link, which currently tracks master.

I'm unsure how the playground works exactly, but ideally every commit would link to its specific version's implementation. If this is not possible, then at least the release versions should link to a correct playground.