Broken links to examples folder (also in lightgbm_ray)

[mirekphd]

@krfricke I think merging this PR made example files much harder to find via the remote web interface i.e. on Github... isn't it because symlinks work only locally?

Resulting empty folders (due to failing symlinks):
https://github.com/ray-project/xgboost_ray/blob/master/examples
https://github.com/ray-project/lightgbm_ray/blob/master/examples
https://github.com/ray-project/lightgbm_ray/blob/main/examples

Resulting missing file (HTTP 404):
https://github.com/ray-project/xgboost_ray/blob/main/examples

... vs. correct (albeit redundantly nested) ones:
https://github.com/ray-project/lightgbm_ray/tree/main/lightgbm_ray/examples
https://github.com/ray-project/xgboost_ray/tree/master/xgboost_ray/examples