[docs] Update LambdaRank example documentation (#2548)
[docs] Update LambdaRank example documentation (#2548)

- Add reference to documentation on the query data format.
- Refer readers to the official install instructions.
- Change command to use relative path to the `lightgbm` binary built at
the project's root when following build instructions.
microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM