doc: add camera_manipulation example project

examples/flutter/camera_manipulation/android/.gitignore

@@ -0,0 +1,13 @@
+gradle-wrapper.jar
+/.gradle
+/captures/
+/gradlew
+/gradlew.bat
+/local.properties
+GeneratedPluginRegistrant.java
+
+# Remember to never publicly share your keystore.
+# See https://flutter.dev/to/reference-keystore
+key.properties
+**/*.keystore
+**/*.jks