Improve section and sub-section titles

[asavageiv]

There is room for improvement of the section titles to reduce redundancy. We don't need to repeat that it's for AAPS everywhere. We're on the AAPS documentation page. We know what it's for.

For example:
Setting Up AAPS becomes Setting Up
Building AAPS becomes just Building or is clarified to Building the APK

Other improvements might be better titles. E.g. the "Change Language" section has both a "Change Language" and "Change Version" sub-section. It would make more sense for the top-level chapter/section to be named "Using the docs" with the sub-sections as they are.