Update index.rst (#4029)
Update index.rst (#4029)

Add alt text to logo image

Co-authored-by: James Lamb <4020df120d2dda32358fb55c9c66975f395dcd8d@gmail.com>
microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM