docs(readme): replace inline sections with doc links

Move the Building section into docs/BUILDING.md and collapse Building,
Architecture, and Roadmap into a compact Documentation block linking to
the dedicated docs, keeping the README focused on what/install/support.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/BUILDING.md

@@ -0,0 +1,24 @@
+# Building from source
+
+Calendula builds with the standard Android Gradle toolchain — no extra setup
+beyond the SDK and a JDK.
+
+## Requirements
+
+- **Android SDK 36+**
+- **JDK 17** — if your default JDK is not 17, set `JAVA_HOME` explicitly.
+
+The Gradle wrapper is checked in, so you don't need a system Gradle.
+
+## Common tasks
+
+```bash
+./gradlew assembleDebug   # debug APK → app/build/outputs/apk/debug/
+./gradlew test            # JVM unit tests
+./gradlew lint            # Android lint
+```
+
+## Release builds
+
+Release signing and the publishing pipeline are documented separately in
+[RELEASING.md](RELEASING.md).