LightGBM

[ci][R-package][python] suppress warning about empty 'compute' director, fix Rtools installation and temporary pin Sphinx version (#3277)

* [R-package] suppress warning about empty 'compute' directory

* get more logs from Rtools install

* empty commit

* add a timeout

* trying Invoke-WebRequest instead

* switch to CRAN link

* do not use Start-Process

* relative paths

* trying suppressmessageboxes

* switch strategy for running Rtools

* Update requirements_rtd.txt

* Update requirements_base.txt

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>