Adding AAPS build instructions for the command line

[je-l]

Hi, thanks for the superb AAPS docs.

For most users, the Android Studio build instructions and the new browser build are already great, but for people familiar with the command line, the experience could be improved with CLI build instructions.

The main benefit is that the build process is simpler.

In a nutshell, it's:

1. Installing Java, git and Android command line tools
2. Compiling AAPS with the gradlew script.
3. Signing the APK with apksigner

I can sketch a PR for adding a third build instruction page. What do you think?