[Docs] Add app logo to main page + cleanup (#7471)

* Add app logo to main page

* rename pages folder

* Add link + cleanup

* Cleanup file extension patterns

* Update Doxygen README.md

doc/doxygen/pages/user_documentation/settings/settings_folder.md

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+@page settings_folder Settings Folder and Backups
+
+Cockatrice keeps all of its preferences in a settings folder on your computer, one `.ini` file per
+group of settings (appearance, sound, decks, and so on). Open it from the main window via
+Help → Open Settings Folder.
+
+# The Per-File Migration
+
+Older versions of Cockatrice stored every setting in a single `global.ini` file. The settings were
+split into per-group `.ini` files so that unrelated groups no longer share one file. The first time
+you start a version that uses the new layout, your settings are moved into the new files
+automatically and nothing is lost. The old `global.ini` is kept next to the new files as
+`global.ini.old`.
+
+# Automatic Backups
+
+Both migrations overwrite files in the settings folder in place, so before either one runs,
+Cockatrice copies the whole folder to a timestamped backup folder:
+
+```
+<data folder>/settings-backup-20261002-193000/
+```
+
+The backup folder sits next to the settings folder, not inside it. Every backup is kept, so if
+settings ever end up wrong you can compare against, or copy files back from, the earlier snapshot.
+Backups are only created when a migration actually has work to do, so a normal start creates
+nothing.
+
+# Downgrading
+
+Settings migrations only run forwards. If you ever go back to an older version of Cockatrice, that
+version does not read the new per-group files, so it will start with default settings.
+
+Before downgrading, copy your settings folder somewhere safe, for example:
+
+-   Linux: `~/.local/share/Cockatrice/settings/`
+-   macOS: `~/Library/Application Support/Cockatrice/settings/`
+-   Windows: `%LOCALAPPDATA%\Cockatrice\settings\`
+
+Portable installs keep the settings folder in the `data` folder next to the executable.
+
+Copying that folder back after downgrading restores your settings. The automatic backup folder from
+the migration is there as a second copy of the same data if you need it.