Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/config/README.md

@@ -0,0 +1,3 @@
+# config.json
+
+This is the system configuration file for your Mattermost server. Settings are specific to different editions of Mattermost. Please read the documentation before making changes: https://docs.mattermost.com/configure/configuration-settings.html/