Add page about arrays

[gkaf89]

The array page is organized according 2 ideas:

• what is the job submission rates that arrays can support, and
• how job array tasks are scheduled and executed.

[jdelguerrero]

Looks great. Just a couple of minor comments:

• Line 13: check the formatting of '-a', right now there is a line break between '-' and 'a'
• Maybe we can change the option arguments order. We can include <start_index>, <end_index> and <increment> explanation before the specific examples,