docs: add troubleshooting for common build failures

- Document that npx cap sync fails in plugin development projects (expected behavior)
- Add troubleshooting for incorrect Gradle project names
- List available Gradle projects and recommend :plugin for builds
- Clarify that www/ directory missing is normal for plugin projects

This addresses the specific failures seen in the terminal output and
provides clear guidance for future users.

BUILDING.md

@@ -505,6 +505,14 @@ rm -rf ~/.gradle/caches/
 # Solution: Build the plugin module instead of the test app
 ./gradlew :plugin:assembleRelease
 
+# Problem: Build fails with "Cannot locate tasks that match ':android:assembleRelease'"
+# Solution: Use correct project name - available projects are:
+# - :app (test app, may have Capacitor issues)
+# - :plugin (plugin library, recommended)
+# - :capacitor-android
+# - :capacitor-cordova-android-plugins
+./gradlew :plugin:assembleRelease
+
 # Problem: Build fails with "Plugin with id 'kotlin-android' not found"
 # Solution: The plugin module doesn't need Kotlin plugin for Java/Kotlin code
 # Check the plugin/build.gradle file
@@ -518,6 +526,11 @@ rm -rf ~/.gradle/caches/
 # - npx cap update  
 # - npx cap add android
 
+# ⚠️  NOTE: npx cap sync will fail in plugin development projects!
+# This is expected - plugin projects don't have www/ directory
+# Error: "Could not find the web assets directory: ./www"
+# This is normal for plugin development projects
+
 # To fix after Capacitor operations:
 ./scripts/fix-capacitor-build.sh