[ci][docs] explicitly recuire fresh versions of RTD stuff (#3315)
[ci][docs] explicitly recuire fresh versions of RTD stuff (#3315)

* bump RTD theme version

* Update requirements_rtd.txt
henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM