Document in places that mention IRC that Slack also exists and has lots of people actively using it

We currently have a really bad community fragmentation problem, in that most of the community has switched over to Slack, and a lot of users also on Matrix, Telegram, Gitter and other protocols, with a remainder still on IRC, some steadfastly so. This has come up several times as an impediment to collaboration, because while we used to all be in #drupal-contribute on IRC, nowadays IRC-based initiative teams end up getting cut off from committers and others who they need to help shepherd their work through who hang out on Slack, and if you don’t know that, you’re left with no recourse but to email people. :

So. Let’s at least make the effort to notify people that Slack exists and is a thing they may want to (also) join.

https://www.drupal.org/irc needs a cross-link to https://www.drupal.org/slack
https://www.drupal.org/slack needs an update with the multitude of channels (esp. “official” initiative channels) available
– Probably other places.

From https://www.drupal.org/node/2490332:

From https://www.drupal.org/node/2885060:

IRC server #room SLACK server #room MATRIX #room:server
freenode #drupal (328) drupal #general (3834) #drupal:matrix.org
freenode #drupal-contribute (204) drupal #contrib (346) #drupal-contribute:matrix.org
freenode #drupal-support (195) drupal #support (3452) #drupal-support:matrix.org
freenode #drupal-accessibility (4) drupal #accessibility (195) #drupal-accessibility:matrix.org
freenode #drupal-commerce (61) drupal #commerce (326) #drupal-commerce:matrix.org
freenode #drupal-console (2) drupal #drupal-console (149) #drupal-console:matrix.org
freenode #drupalcon (21) drupal #drupalcon (253) #drupal-drupalcon:matrix.org
freenode #drupal-twig (19) drupaltwig #twig (912) #drupal-twig:matrix.org
freenode #drush (42) #drupal-drush:matrix.org
freenode #aegir (33) drupal #aegir (16) #drupal-aegir:matrix.org
freenode #drupal-media (54) drupal #media (88) #drupal-media:matrix.org
freenode #drupal-search-api (16) drupal #search (60) #drupal-search:matrix.org
freenode #drupalorg (25) drupal #drupalorg (5) #drupal-drupalorg:matrix.org
freenode #drupal-vm (33) #drupal-vm:matrix.org
freenode #drupal-be (3) drupalbe #general (32) #drupal-belgium:matrix.org
freenode #drupal-pt drupal #drupal-portugal #drupal-portugal:matrix.org


Source: https://www.drupal.org/project/issues/rss/documentation