LightGBM

[docs] 🎨 Sphinx Autosummary for generating Python-API documentation (#2286)

* 🎨 `sphinx.ext.autosummary` for generating Python-API summaries

Add `docs/.gitignore` to not track autosummary stubs
Add `sphinx.ext.autosummary` in `docs/conf.py`
  Add 'members' and 'inherited-members' as default parameters
  Add 'autosummary = True' for setting output with `:toctree:`
Add `.. autosummary::` tags to replace `.. autoclass::`

Previously the `Python-API.rst` dumped all of the Python API onto
a single page.

This replaces the Python-API documentation with an index listing
all modules, and paginates all functions and classes onto
separate pages.

* ✏️ Corrections following feedback

Drop `docs/.gitignore` to use the general `.gitignore`
Add `show-inheritance` to `autodoc_default_flags` in `docs/conf.py`
Fix `both` to `class` in `autoclass_content` in `docs/conf.py`

* ✏️ Replacing deprecated Sphinx parameter

Fix deprecated `autodoc_default_flags` to `autodoc_default_options`

* ✏️ Adding `autodoc_default_flags` in to support early Sphinx versions

Add `autodoc_default_flags` with parameters from
  `autodoc_default_options`