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

webapp/platform/client/README.md

@@ -0,0 +1,14 @@
+# Mattermost Client
+
+This package contains the JavaScript/TypeScript client for [Mattermost](https://github.com/mattermost/mattermost-server). It's used by [the Mattermost web app](https://github.com/mattermost/mattermost-webapp) and related projects.
+
+## Compilation and Packaging
+
+As a member of Mattermost with write access to our NPM organization, you can build and publish this package by running the following commands:
+
+```bash
+npm run build --workspace=packages/client
+npm publish --workspace=packages/client
+```
+
+Make sure to increment the version number in `package.json` first! You can add `-0`, `-1`, etc for pre-release versions.