[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.