[MLIR][NVVM] Move docs to correct folder

Author: grypp

mlir/docs/Dialects/NVVM/_index.md renamed to mlir/docs/Dialects/NVVMDialect.md

@@ -81,4 +81,7 @@ introduce high-level ops that expand into multiple unrelated NVVM intrinsics or
 that lower to no intrinsic at all. Such abstractions belong in higher-level
 dialects (e.g., `nvgpu`, `gpu`, or project-specific dialects). The design
 intent is a thin, predictable, low-level surface with near-mechanical lowering
-to NVVM/LLVM IR.
+to NVVM/LLVM IR.
+
+[TOC]
+