Final clean up API features

[LuisMossburger]

Description Concerns #152

This changes contain clean ups mainly from API related classes. This means in detail:

• some blank lines were inserted for better readability.
• getter methods (like getContext()) have been replaced by require ones wherever possible to avoid NullPointerExceptions.
• comments have been changed in format, content or type (all method comments are JavaDoc comments now), added for more complex but private methods as well.
• imports and class variables have been optimized in sorting.
• code has been simplified by declaring the mainActivity as context once and reuse the class variable instead of calling the requireContext() or requireActivity() (or according getter methods) multiple times within a class.

This PR also updates the README for the Final Release! 💪

Affects API related classes.

Notes for Reviewer Please review & comment/approve. Thanks!

[LuisMossburger]

Thanks for the review, applied all of your suggestions except the "Icons by Freepik" as this is a standard text that is preferred by Flaticon :)