[R-package] plots are broken in R documentation

[jameslamb]

See #3270 (comment) and the comments that follow it.

The plots on these pages are broken:

• https://lightgbm.readthedocs.io/en/latest/R/reference/lgb.plot.importance.html
• https://lightgbm.readthedocs.io/en/latest/R/reference/lgb.plot.interpretation.html

This might be the result of the fact that we use a very old R version (3.5.1) in build in the site. See docs/conf.py.

[github-actions]

This issue has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.