Developer manual: what to clean up on uninstall in repair.rst

[miaulalala]

developer_manual/digging_deeper/repair.rst explains the uninstall repair step, but not what an app should actually clean up, and the app guidelines require it.

• Add a short checklist of what to remove on uninstall: app config values, user preferences, background jobs, appdata files, and (if wanted) database tables.
• Add a note on downgrades: what happens to migrations and data when an older app version is installed.