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>

README.md

@@ -101,31 +101,11 @@ rebuilds on F-Droid's own schedule. Add it for the freshest builds:
 Both channels share the same signing key, so you can switch between them
 without reinstalling. Or build from source — see below.
 
-## 🛠 Building
+## 📚 Documentation
 
-Requires Android SDK 36+ and JDK 17. The Gradle wrapper is checked in:
+- **[Building from source](docs/BUILDING.md)** — requirements and Gradle tasks
-
+- **[Architecture](docs/ARCHITECTURE.md)** — the layered design and key pipelines
-```bash
+- **[Roadmap](.planning/ROADMAP.md)** — what's shipped and what's next
-./gradlew assembleDebug   # debug APK
-./gradlew test            # JVM unit tests
-./gradlew lint            # Android lint
-```
-
-If your default JDK is not 17, set `JAVA_HOME` explicitly.
-
-## 🏗 Architecture
-
-Single-activity Compose app, layered `UI → Repository → DataSource →
-CalendarContract`, observer-driven refresh, JVM-first tests. The full tour —
-including the recurring-write and reminder pipelines — lives in
-[docs/ARCHITECTURE.md](docs/ARCHITECTURE.md).
-
-## 🗺 Roadmap
-
-Shipped: read (v1.0), write (v1.1–v2.0), reminder delivery (v1.4).
-Next up: power-user features — widget, search, tablet layouts. The living
-roadmap is in [.planning/ROADMAP.md](.planning/ROADMAP.md), the release
-history in [CHANGELOG.md](CHANGELOG.md).
 
 ## 📜 License
 

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).