add Flatpak migration instruction (copied from News post https://mixxx.org/news/2024-02-21-official-flatpak-now-available-on-flathub/)

Author: ronso0

Diff for: source/chapters/advanced_topics.rst

@@ -271,3 +271,18 @@ audio decoders may have changed which may cause beatgrids and cue points to appe
 set off from the desired points in the audio stream. Unfortunately, there is no automatic fix available, yet.
 As of now you need to shift cues for each track individually with the cue shift buttons in the beatgrid editing
 controls section, see :ref:`interface-waveform`.
+
+.. _advanced-migrate-flatpak
+
+Migrate your Mixxx library and settings to Flatpak
+==================================================
+
+Since version 2.5 Mixxx is also provided as Flatpak.
+
+If you do want to switch from a distribution package or Mixxx built from source code to the Flatpak, a little setup is required. Flatpak stores Mixxx's database, settings, and custom controller mappings in a different location than the `~/.mixxx` directory that Mixxx has always used before. Flatpak uses `~/.var/app/org.mixxx.Mixxx/.mixxx` instead. To copy your database, settings, and controller mappings into the Flatpak sandbox, run:
+::
+
+  mkdir -p ~/.var/app/org.mixxx.Mixxx
+  cp -r ~/.mixxx ~/.var/app/org.mixxx.Mixxx
+
+If your library contains any directories outside of your XDG Music Directory (`~/Music` unless you have reconfigured this), you will need to relink these directories to access the files inside of Flatpak's sandbox. Run Mixxx from the Flatpak and go to :menuselection:`Options --> Preferences --> Library`. Any directories that have a warning icon with `!` in a yellow triangle need to be relinked. Select the directory in the preferences window and click the Relink button, then select the directory in the file picker dialog. Flatpak will automatically remap the directory under `/run/user` so Mixxx can read and write your music files from inside the Flatpak sandbox. Press Ok in the preferences window, then in Mixxx's main window, go to :menuselection:`Library > Rescan Library`.