[docs] limit number of files to scan for autosummary generation and reorganize conf.py (#2297)
[docs] limit number of files to scan for autosummary generation and reorganize conf.py (#2297)

* limit number of files to scan for autosummary generation

* reorganize conf.py

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