Reformat README and the wiki

[Renarde-dev]

Documentation is all over the place and need to be better organised.

List of task may be expended later

Rewriting the README

• Remove usage guide
• Point to official user guide (https://doc.piwigo.org/mobile-apps/piwigo-ng-android-mobile-app)
• Add link to wiki
• Link to contributing
• Fdroid/IzzyDroid and Google play badges

Reorganising the wiki

• Remove references to iOS
• Dedicated architecture page
• Dedicated Crowdin section ?
• Remove flutter get started tutorial and link to official doc instead
• Same for the release process, official documentation and keep to the specifics of piwigo NG
• Move ? or remove the features section
• Add an API reference section
• Add a fastlane section (https://gitlab.com/IzzyOnDroid/repo/-/wikis/Fastlane)