Update 00-getting-started.mdx

Author: adrw

docs/docs/guides/building-a-tab/00-getting-started.mdx

@@ -1,3 +1,9 @@
+# Deprecation Warning
+
+Misk-Web is deprecated, please do not create new tabs and plan to migrate off long term.
+
+The Misk dashboard is migrating away from a React Typescript stack to using server-side-rendered kotlinx.html using Hotwire and Tailwind CSS.
+
 # Getting Started
 
 Here's how to make a new Misk-Web tab in the current directory.