[docs] Remove Gitter and Slack (fixes #3689) (#3710)
[docs] Remove Gitter and Slack (fixes #3689) (#3710)


microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM