Portable Playlist - User manual + Technical documentation

RetroArch has a so called Portable Playlist feature.

• But it is totally undocumented!

  • Only a few rumors and bad guesses on some user forums.
  • And 3 issues in the issue tracker and a few related pull requests, which only talk about some very particulars
  • But the official documentation contains nothing on it
  • But nowhere a wholistic documentation

• It should be in the official documentation, ideally at roms-playlists-thumbnails.md and decribe:

  • How this shall work from the user perspective (in the GUI and when manually creating .LPL files)
    • And the maintenance advantages when moving those portable playlists between systems
      • with different filepath conventions (Windows, Android, macOS)
      • and/or different device/drive/folder structure.
  • And how it works on a technical level:
    • The interplay between each items[] → path and the base_content_directory in the root of the LPL.
    • I also saw examples where base_content_directory can not only take an absolute path (the base path) but seemingly can also contain some flags like M|, !r, W{, but nothing of this is documented.
  • So before spending hours and days of trial and error, I'd really appreciate if some of the involved developers could document this feature.

Start with docs/guides/roms-playlists-thumbnails.md and review the three linked Portable Playlist issues and related pull requests for existing details. Document GUI and manual .LPL usage, portability across path conventions and device structures, and the relationship between items[].path and root-level base_content_directory, including the mentioned flags. Done means the official documentation provides a coherent user and technical explanation.