meta: add links to OpenJSF Slack

The OpenJS Foundation has an official Slack, and the Node.js project has
many channels there for working groups as well as for general
discussions. Add links to the workspace and channels for folks who want
to join.

PR-URL: nodejs#35128
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

.github/SUPPORT.md

@@ -15,6 +15,7 @@ If you didn't find an answer in the resources above, try these unofficial
 resources:
 
 * [Questions tagged 'node.js' on Stack Overflow](https://stackoverflow.com/questions/tagged/node.js)
+* [#nodejs](https://openjs-foundation.slack.com/archives/CK9Q4MB53) channel on the OpenJS Foundation Slack ([join here](https://slack-invite.openjsf.org/))
 * [#node.js channel on chat.freenode.net](https://webchat.freenode.net?channels=node.js&uio=d4)
 * [Node.js Slack Community](https://node-js.slack.com/)
   * To register: [nodeslackers.com](https://www.nodeslackers.com/)

doc/guides/contributing/pull-requests.md

@@ -46,7 +46,16 @@ Node.js. We cannot accept such patches.
 In case of doubt, open an issue in the
 [issue tracker](https://github.com/nodejs/node/issues/) or contact one of the
 [project Collaborators](https://github.com/nodejs/node/#current-project-team-members).
-Node.js has two IRC channels:
+
+Node.js has many channels on the
+[OpenJS Foundation Slack](https://slack-invite.openjsf.org/). Interesting
+channels are:
+[#nodejs](https://openjs-foundation.slack.com/archives/CK9Q4MB53) for general
+help, questions and discussions.
+[#nodejs-dev](https://openjs-foundation.slack.com/archives/C019Y2T6STH) for
+development of Node.js core specifically.
+
+Node.js also has two IRC channels:
 [#Node.js](https://webchat.freenode.net/?channels=node.js) for general help and
 questions, and
 [#node-dev](https://webchat.freenode.net/?channels=node-dev) for development of

onboarding.md

@@ -56,11 +56,15 @@ onboarding session.
   * Watching the main repo will flood your inbox (several hundred notifications
     on typical weekdays), so be prepared
 
-* `#node-dev` on [webchat.freenode.net](https://webchat.freenode.net/) is the
-  best place to interact with the TSC / other Collaborators
-  * If there are any questions after the session, a good place to ask is there!
-  * Presence is not mandatory, but please drop a note there if force-pushing to
-    `master`
+The project has two venues for real-time discussion:
+* [`#nodejs-dev`](https://openjs-foundation.slack.com/archives/C019Y2T6STH) on
+  the [OpenJS Foundation](https://slack-invite.openjsf.org/)
+* `#node-dev` on [webchat.freenode.net](https://webchat.freenode.net/) is a
+  great place to interact with the TSC and other Collaborators
+  * If there are any questions after the session, a good place to ask is
+    there!
+  * Presence is not mandatory, but please drop a note there if force-pushing
+    to `master`
 
 ## Project goals & values