[SPARK-11381][DOCS] Replace example code in mllib-linear-methods.md using include_example

## What changes were proposed in this pull request?

This PR replaces example codes in `mllib-linear-methods.md` using `include_example`
by doing the followings:
  * Extracts the example codes(Scala,Java,Python) as files in `example` module.
  * Merges some dialog-style examples into a single file.
  * Hide redundant codes in HTML for the consistency with other docs.

## How was the this patch tested?

manual test.
This PR can be tested by document generations, `SKIP_API=1 jekyll build`.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11320 from dongjoon-hyun/SPARK-11381.