[SPARK-18923][DOC][BUILD] Support skipping R/Python API docs

## What changes were proposed in this pull request?

We can build Python API docs by `cd ./python/docs && make html for Python` and R API docs by `cd ./R && sh create-docs.sh for R` separately. However, `jekyll` fails in some environments.

This PR aims to support `SKIP_PYTHONDOC` and `SKIP_RDOC` for documentation build in `docs` folder. Currently, we can use `SKIP_SCALADOC` or `SKIP_API`. The reason providing additional options is that the Spark documentation build uses a number of tools to build HTML docs and API docs in Scala, Python and R. Specifically, for Python and R,

- 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.

## How was this patch tested?

Manual.

**Skipping Scala/Java/Python API Doc Build**
```bash
$ cd docs
$ SKIP_SCALADOC=1 SKIP_PYTHONDOC=1 jekyll build
$ ls api
DESCRIPTION R
```

**Skipping Scala/Java/R API Doc Build**
```bash
$ cd docs
$ SKIP_SCALADOC=1 SKIP_RDOC=1 jekyll build
$ ls api
python
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #16336 from dongjoon-hyun/SPARK-18923.

docs/README.md

@@ -69,4 +69,5 @@ may take some time as it generates all of the scaladoc.  The jekyll plugin also
 PySpark docs using [Sphinx](http://sphinx-doc.org/).
 
 NOTE: To skip the step of building and copying over the Scala, Python, R API docs, run `SKIP_API=1
-jekyll`.
+jekyll`. In addition, `SKIP_SCALADOC=1`, `SKIP_PYTHONDOC=1`, and `SKIP_RDOC=1` can be used to skip a single
+step of the corresponding language.

docs/_plugins/copy_api_dirs.rb

@@ -113,36 +113,41 @@ if not (ENV['SKIP_API'] == '1')
     File.open(css_file, 'a') { |f| f.write("\n" + css.join()) }
   end
 
-  # Build Sphinx docs for Python
+  if not (ENV['SKIP_PYTHONDOC'] == '1')
+    # Build Sphinx docs for Python
 
-  puts "Moving to python/docs directory and building sphinx."
-  cd("../python/docs")
-  system("make html") || raise("Python doc generation failed")
+    puts "Moving to python/docs directory and building sphinx."
+    cd("../python/docs")
+    system("make html") || raise("Python doc generation failed")
 
-  puts "Moving back into home dir."
-  cd("../../")
+    puts "Moving back into docs dir."
+    cd("../../docs")
 
-  puts "Making directory api/python"
-  mkdir_p "docs/api/python"
+    puts "Making directory api/python"
+    mkdir_p "api/python"
 
-  puts "cp -r python/docs/_build/html/. docs/api/python"
-  cp_r("python/docs/_build/html/.", "docs/api/python")
+    puts "cp -r ../python/docs/_build/html/. api/python"
+    cp_r("../python/docs/_build/html/.", "api/python")
+  end
 
-  # Build SparkR API docs
-  puts "Moving to R directory and building roxygen docs."
-  cd("R")
-  system("./create-docs.sh") || raise("R doc generation failed")
+  if not (ENV['SKIP_RDOC'] == '1')
+    # Build SparkR API docs
 
-  puts "Moving back into home dir."
-  cd("../")
+    puts "Moving to R directory and building roxygen docs."
+    cd("../R")
+    system("./create-docs.sh") || raise("R doc generation failed")
 
-  puts "Making directory api/R"
-  mkdir_p "docs/api/R"
+    puts "Moving back into docs dir."
+    cd("../docs")
 
-  puts "cp -r R/pkg/html/. docs/api/R"
-  cp_r("R/pkg/html/.", "docs/api/R")
+    puts "Making directory api/R"
+    mkdir_p "api/R"
 
-  puts "cp R/pkg/DESCRIPTION docs/api"
-  cp("R/pkg/DESCRIPTION", "docs/api")
+    puts "cp -r ../R/pkg/html/. api/R"
+    cp_r("../R/pkg/html/.", "api/R")
+
+    puts "cp ../R/pkg/DESCRIPTION api"
+    cp("../R/pkg/DESCRIPTION", "api")
+  end
 
 end