[SPARK-20015][SPARKR][SS][DOC][EXAMPLE] Document R Structured Streaming (experimental) in R vignettes and R & SS programming guide, R example

## What changes were proposed in this pull request?

Add
- R vignettes
- R programming guide
- SS programming guide
- R example

Also disable spark.als in vignettes for now since it's failing (SPARK-20402)

## How was this patch tested?

manually

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #17814 from felixcheung/rdocss.