After #400, the gazelle plugin has been cleanly separated out into its own bazel workspace, which will soon finally allow us to mark it stable. But this means:
* we need to change our bazelci config to explicitly build and test it, since `bazel build //...` no longer includes the plugin;
* we need to add proper distribution rules for it;
* we need to update release instructions, since now we will have two distribution tarballs
