Add documentation for NCCL NET plugins

Also repurpose dummy plugin as example, including headers and
compat layers from v6 to v2.


[ROCm/rccl commit: 55b1d8ab98]

projects/rccl/ext-net/example/nccl/err.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2017-2022, NVIDIA CORPORATION. All rights reserved.
+ */
+
+#ifndef NCCL_ERR_H_
+#define NCCL_ERR_H_
+
+/* Error type for plugins */
+typedef enum { ncclSuccess                 =  0,
+               ncclUnhandledCudaError      =  1,
+               ncclSystemError             =  2,
+               ncclInternalError           =  3,
+               ncclInvalidArgument         =  4,
+               ncclRemoteError             =  6 } ncclResult_t;
+
+#endif