Docs: add a list with available external standards to the wiki

[jrfnl]

Repost from squizlabs/PHP_CodeSniffer#1165:

I was looking for a list of coding standards of well-known open source projects with an eco-system of add-on projects (plugins, themes), such as CakePHP, Drupal, Joomla, Magento, TYPO3, WordPress, preferably with links to the repo and possible link to a documented descriptive standard.

So far I have not been able to find such a list (found quite some standards, but took me hours).

Would it be an idea to add such a list of externally maintained standards which can be used with PHPCS as a wiki page to this repo ?

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👉🏻 The thread in the original post contains lots of links to external standards and would be a good starting point.