additions (from f-droid) and some link updates

entries/retro_stack.md

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+# Retro Stack
+
+- Home: https://cosmin.hume.ro/project/retro-stack-gdx/
+- State: mature
+- Download: https://f-droid.org/de/packages/ro.hume.cosmin.retrostack/
+- Platform: Android
+- Keyword: arcade, skill
+- Code repository: https://codeberg.org/cosmin/retro-stack-gdx.git
+- Code language: Kotlin
+- Code license: GPL-3.0
+- Code dependency: libGDX
+
+## Building
+
+- Build system: Gradle