Update documentation from Windows/Android testing

[TheFloatingString]

• ImageMagick Legacy build
• Windows "./gradlew" bundleRelease must be done in android directory
• Keystore documentation (TBC)
• Alternative setup of environment variables in gradle build file
• Add react-native run-android --variant=release
• Reminder for .env variables or turning on backend server

Common issues:

• armeabi-v7a not found: try re-running
• imagemagick: install legacy edition
• Package com.msucongress.mariapp signatures do not match the previously installed versions: get correct keystore credentials