Docs: Add documentation for WordPress.Security.EscapeOutput

[brentwilson-clariio]

Related to #1722

Adds WordPress/Docs/Security/EscapeOutputStandard.xml with short description and invalid/valid examples (echo, printf).
Tested with:
vendor/bin/phpcs --standard=WordPress --generator=Text --sniffs=WordPress.Security.EscapeOutput

[rodrigoprimo]

Thanks for working on this PR, @brentwilson-clariio!

I haven't done a full review yet of this PR, but I have some initial remarks:

• Could you please update this PR to apply all the applicable points that were raised in the review of #2591? Since that PR was reviewed first, it might be a good idea to wait until it is finalized before updating this PR, as more points discussed there might be relevant here as well.
• Per the description in #1722, verifying calls for addError()/addWarning() helps identify what the sniff is checking. There are some exceptions, but typically the documentation will contain one <standard> and one <code_comparison> block per error/warning message. This sniff raises a few errors that are not covered in this PR, and I believe they should. For example:

  WordPress-Coding-Standards/WordPress/Sniffs/Security/EscapeOutputSniff.php

  Lines 662 to 666 in afcb17e

  	$this->phpcsFile->addError(
  	'All output should be run through an escaping function (see the Security sections in the WordPress Developer Handbooks), found interpolation in unescaped heredoc.',
  	$current,
  	'HeredocOutputNotEscaped'
  	);

[rodrigoprimo]

@brentwilson-clariio, I was just wondering if you'll have a chance to finish this off in the near future. It would be great if this PR could be included in the next WPCS release.

If you haven't got time or lost interest, please let us know and we'll see if we can find someone to take over. Thanks!

[rodrigoprimo]

@brentwilson-clariio, please let us know within a week if you are still interested in finishing this PR. If we don't hear back from you, we will presume you don't have time, and we will see if we can find someone else to take over and finish it. Thanks for your work so far!