Make the tutorial more user-friendly by adding `build.gradle` files

The Checker Framework manual says that no installation is necessary because you will probably use the Checker Framework with a build system such as Maven or Gradle. Then, the tutorial immediately asks users to download and install the Checker Framework.
(Parts of the tutorial use Ant. This still requires the Checker Framework to be downloaded.)

We should change the tutorial:

• create build.gradle build files, one per example in the tutorial
• change the tutorial instructions so the command to run is ./gradlew, not javac or ant
• remove the instructions to install the Checker Framework
• remove uses of "from the command line" throughout, because it is no longer necessary to make a distinction with the Eclipse version. The instructions can say that a user may run the tutorial in an IDE.