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'
  	);