Prepare CLI for move into mainline

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

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+# Build
+
+The `dioxus build` command can help you `pack & build` a dioxus project.
+
+```
+dioxus-build 
+Build the Rust WASM app and all of its assets
+
+USAGE:
+    dioxus build [OPTIONS]
+
+OPTIONS:
+        --example <EXAMPLE>      [default: ""]
+        --platform <PLATFORM>    [default: "default_platform"]
+        --release                [default: false]
+```
+
+You can use this command to build a project:
+
+```
+dioxus build --release
+```
+
+## Target platform
+
+Use the `platform` option to choose your target platform:
+
+```
+# for desktop project
+dioxus build --platform desktop
+```
+
+`platform` currently only supports `desktop` & `web`.
+
+```
+# for web project
+dioxus build --platform web
+```
+
+## Build Example
+
+You can use the `example` option to select a example to build:
+
+```
+# build the `test` example
+dioxus build --exmaple test
+```