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

[StrikerRUS]

Refer to #2286 (comment).

Before:

D:\Users\nekit\Downloads\LightGBM\docs>make html
Running Sphinx v1.8.5
making output directory...
[autosummary] generating autosummary for: Advanced-Topics.rst, C-API.rst, Development-Guide.rst, Experiments.rst, FAQ.rst, Features.rst, GPU-Performance.rst, GPU-Targets.rst, GPU-Tutorial.rst, GPU-Windows.rst, Installation-Guide.rst, Parallel-Learning-Guide.rst, Parameters-Tuning.rst, Parameters.rst, Python-API.rst, Python-Intro.rst, Quick-Start.rst, README.rst, gcc-Tips.rst, index.rst

...

Now:

D:\Users\nekit\Downloads\LightGBM\docs>make html
Running Sphinx v1.8.5
making output directory...
[autosummary] generating autosummary for: Python-API.rst

...

@hayesall Can you please give your review?

UPD: Also, group options in conf.py logically.

[hayesall]

The only change I would suggest is moving these lines back below the templates_path variable to keep the diffs smaller.

[StrikerRUS]

@hayesall Thanks for your review! I moved autosummary_generate variable with the aim to keep logic grouping: autodoc_* variables are closer to autosummary_* and templates_path is closer to source_suffix, so the first group is about auto_ extensions and the second one is about source file paths.

IDK, maybe you're right and smaller diff is more important than grouping. Let's wait for one more opinion from another reviewer...

[hayesall]

@StrikerRUS Logical ordering is more important small diffs, I like this now that I see it this way 👍

While we're on the subject: it doesn't appear that templates are used in this repository, so we could drop the templates_path variable.

[StrikerRUS]

@hayesall Good suggestion! Thanks! I also dropped commented option and reorganized two other options connected to extensions in the latest commit 98ebbd8.