LightGBM

[ci][docs] generate docs for C API (#2059)

* use file to install deps for docs

* added C_API docs

* use breathe without exhale

* added missed params descriptions and make Doxygen fail for warnings

* escape char hotfix

* ignore unknown directive for rstcheck

* better handle env variable

* hotfix for 'Unknown directive type' error with C_API=NO

* Update .gitignore

* fixed pylint

* use already defined constants in conf.py

* do not suppress Doxygen's output

* addressed review comments

* removed unneeded import