Prepare CLI for move into mainline

packages/cli/docs/src/cmd/clean.md

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+# Clean
+
+`dioxus clean` will clear the build artifacts (the out_dir and the cargo cache)
+
+```
+dioxus-clean 
+Clean build artifacts
+
+USAGE:
+    dioxus clean
+```
+
+# Example
+
+```
+dioxus clean
+```
+