Add documentation for parallel keyword about how to split a file

Problem to solve

The parallel keyword is missing a generic example of how you might split a file into multiple parts, causing some people maybe to not realize what's possible there.

Further details

From the docs today:

parallel allows you to configure how many instances of a job to run in parallel. This value has to be greater than or equal to two (2) and less than or equal to 50.

This creates N instances of the same job that run in parallel. They’re named sequentially from job_name 1/N to job_name N/N.

For every job, CI_NODE_INDEX and CI_NODE_TOTAL environment variables are set.

A simple example:

test:
  script: rspec
  parallel: 5

Proposal

Add an actual script example that splits a file into a list of somethings and runs something on each one (this example is not tested):

process_files:
  parallel: 5
  script:
    - find . -name "*.png" >to_process.txt
    - split -l$((`wc -l < to_process.txt`/$CI_NODE_TOTAL)) to_process.txt
    - cat to_process.txt$CI_NODE_INDEX |xargs do_processing

Who can address the issue

Other links/references