[mlir][gpu] Improve `gpu-lower-to-nvvm-pipeline` Documentation (#77062)

This PR improves the documentation for the `gpu-lower-to-nvvm-pipeline`
(as it was remaning item for #75775)

- Changes pipeline `gpu-lower-to-nvvm` -> `gpu-lower-to-nvvm-pipeline`
- Adds a section in GPU Dialect in website. It clarifies the pipeline's
functionality in lowering primary dialects to NVVM targets.