Update SUMMARY.md

Author: juntyr

src/doc/rustc/src/SUMMARY.md

@@ -84,6 +84,7 @@
     - [wasm32-wasip1](platform-support/wasm32-wasip1.md)
     - [wasm32-wasip1-threads](platform-support/wasm32-wasip1-threads.md)
     - [wasm32-wasip2](platform-support/wasm32-wasip2.md)
+    - [wasm32-unknown-emscripten](platform-support/wasm32-unknown-emscripten.md)
     - [wasm32-unknown-unknown](platform-support/wasm32-unknown-unknown.md)
     - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md)
     - [\*-win7-windows-msvc](platform-support/win7-windows-msvc.md)