Bug 1637983 - Replace Slack links by Matrix in documentation. r=jdescottes.

We take this as an opportunity to remove
outdated information in aboutdebugging readme.

Differential Revision: https://phabricator.services.mozilla.com/D75314

devtools/client/aboutdebugging/README.md

@@ -81,8 +81,6 @@ If you have any questions about the code, features, planning, the active team is
 - engineering: Belén Albeza (:ladybenko)
 - engineering: Daisuke Akatsuka (:daisuke)
 - engineering: Julian Descottes (:jdescottes)
-- engineering: Ola Gasidlo (:ola))
-- engineering management: Soledad Penadés (:sole)
 - product management: Harald Kischner (:digitarald)
 
-You can find us on [Slack](https://devtools-html-slack.herokuapp.com/) or IRC on #devtools at irc.mozilla.org.
+You can find us on [Matrix](https://chat.mozilla.org/#/room/#devtools:mozilla.org).

devtools/docs/getting-started/README.md

@@ -8,7 +8,7 @@ DevTools is a complex web app, but if you're familiar with either HTML/CSS or Ja
 - Set up your dev environment
 - Write the patch and get it reviewed
 
-Feel free to ask questions at any point on the [DevTools Slack](https://devtools-html-slack.herokuapp.com/).
+Feel free to ask questions at any point on [Matrix](https://chat.mozilla.org/#/room/#devtools:mozilla.org).
 
 ## Claim a bug
 
@@ -20,7 +20,7 @@ Claim the bug by creating a Bugzilla account and posting a comment on the bug’
 
 Follow the steps of Firefox’s [contributor guide](https://firefox-source-docs.mozilla.org/contributing/how_to_contribute_firefox.html) to install and run Firefox locally. During installation, follow the steps for “Artifact Mode.” 
 
-If you run into errors about missing libraries, search the web to learn how to install whatever is missing. If you get stuck, ask for help on [Slack](https://devtools-html-slack.herokuapp.com/).
+If you run into errors about missing libraries, search the web to learn how to install whatever is missing. If you get stuck, ask for help on [Matrix](https://chat.mozilla.org/#/room/#devtools:mozilla.org).
 
 ## Write the patch and get it reviewed