[SPARK-15412][PYSPARK][SPARKR][DOCS] Improve linear isotonic regression pydoc & doc build insturctions

## What changes were proposed in this pull request?

PySpark: Add links to the predictors from the models in regression.py, improve linear and isotonic pydoc in minor ways.
User guide / R: Switch the installed package list to be enough to build the R docs on a "fresh" install on ubuntu and add sudo to match the rest of the commands.
User Guide: Add a note about using gem2.0 for systems with both 1.9 and 2.0 (e.g. some ubuntu but maybe more).

## How was this patch tested?

built pydocs locally, tested new user build instructions

Author: Holden Karau <holden@us.ibm.com>

Closes #13199 from holdenk/SPARK-15412-improve-linear-isotonic-regression-pydoc.