Add some diagram of app project

[Magicianred]

Insert some schemes to start having an overview of the app. To evaluate and improve with developers

[dcremonini]

Hi Simone,
thank you for contributing.
Here are my remarks:

• the documentation is in Italian (everything we publish should be in English - this is true for filenames and contents)
• in the diagram I see a mix of Android front-end and back-end mixed together. Maybe this repository is not the best one

Later I will also check the content

[Magicianred]

Thanks @dcremonini for the tips, you're right

these are a first draft to start participating, the architecture is not exactly like the original and in fact I am trying to review it together with a developer who has seen the code and allows me to bring it closer to the real situation

The .drawio files are editable with the app https://app.diagrams.net/ and I invite everyone to make the corrections they believe useful for improvement and to make them more usable for everyone (for example by translating the texts in English)

or if anyone knows of another easy tool for making uml diagrams we can evaluate them