This issue aims to support `SKIP_PYTHONDOC` and `SKIP_RDOC` for documentation generation. Currently, we can use `SKIP_SCALADOC` or `SKIP_API`.
The reason is that the Spark documentation build uses a number of tools to build HTML docs and API docs in Scala, Python and R. Especially,
- Python API docs requires `sphinx`.
- R API docs requires `R` installation and `knitr` (and more others libraries).
In other words, we cannot generate Python API docs without R installation. Also, we cannot generate R API docs without Python `sphinx` installation.
If Spark provides `SKIP_PYTHONDOC` and `SKIP_RDOC` like `SKIP_SCALADOC`, it would be more convenient.