[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/beta.md

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+@page beta_release Beta Release
+
+The beta version of Cockatrice features all the newest features before they make their way into the latest stable
+release.
+It allows us to test these features ahead of time and you to give feedback on them.
+We release new betas very frequently and usually release a follow-up beta with a fix within a day or two if we break
+anything. In contrast, stable releases are released much less frequently but ensured to be stable.
+
+# What to Expect from the Beta
+
+Using the beta and giving feedback on it helps us improve Cockatrice a lot.
+We do not perform any automatic data collection so your voluntary feedback is the only way for us to get a sense of what
+you enjoy, expect, or would like to change.
+
+We highly appreciate your active participation and feedback.
+
+That being said, some words of warning should be issued:
+
+The beta release may contain unfinished features, bugs, or performance issues.
+It is intended for testing and feedback and may be less stable than the default release.
+
+We recommend using the beta only if you are comfortable encountering issues and helping us improve Cockatrice.
+
+# Switching to the Beta
+
+To switch to the Beta, navigate to your settings, using the Tab "Cockatrice → Settings" or Ctrl + Shift + P (default
+shortcut).
+
+Within the settings, in the first tab 'General', the first section 'Personal Settings' contains the setting 'Release
+Channel'.
+
+\image html release_channel_beta.png
+
+Switch this to 'Beta'.
+
+Afterwards, close your settings, and use the 'Help' tab to search for a client update. You should be prompted to install
+the beta version.
+
+# Switching back to Stable
+
+Follow the same steps as above, but set Release Channel to 'Default'.
+
+# Giving Feedback
+
+If you encounter a bug or unexpected behavior while using the beta, please report it on:
+
+- GitHub: https://github.com/Cockatrice/Cockatrice/issues
+- Discord: https://discord.gg/4hNswvHeFd in #beta-testing
+
+When reporting issues, please include:
+
+- Your operating system
+- The beta version number (found in 'Help → About Cockatrice')
+- Steps to reproduce the problem, if possible

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.