[SPARK-18849][ML][SPARKR][DOC] vignettes final check reorg

## What changes were proposed in this pull request?

Reorganizing content (copy/paste)

## How was this patch tested?

https://felixcheung.github.io/sparkr-vignettes.html

Previous:
https://felixcheung.github.io/sparkr-vignettes_old.html



Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #16301 from felixcheung/rvignettespass2.

(cherry picked from commit 38fd163d)
Signed-off-by: Felix Cheung <felixcheung@apache.org>