docs: move changelog to desktop folder

ui/desktop/CHANGELOG.md

@@ -1,10 +1,8 @@
-# Boundary UI CHANGELOG
+# Boundary Desktop CHANGELOG
 
-Canonical reference for changes, improvements, and bugfixes for Boundary UI.
+Canonical reference for changes, improvements, and bugfixes for Boundary Desktop.
 
 
 ## 0.1.0 (2021.02.16)
 
-### Desktop
-
 v1.0.0-alpha is the first release of Boundary Desktop. As a result there are no changes, improvements, or bugfixes from past versions.