Update Linux Distro installation instructions (Flatpak/AUR/Snap)

Perhaps it might be worth mentioning the unofficial and community maintained flatpak, snap and AUR builds of Anki. It might be worth recognizing that they exist and maybe mention that they are not officially maintained by the project.

The unofficial Anki flatpak by Damien Elmes on [Flathub](https://flathub.org/apps/net.ankiweb.Anki) have 230k+ installs as of December 2024. I also use the flatpak version myself for easier updates.

It can probably be inserted with a few paragraphs in this section.

[2. Platform Notes > 2.3 Linux > 2.3.5 Linux Distro Packages](https://docs.ankiweb.net/platform/linux/distro-packages.html)


Thanks!

Open the linked Platform Notes > Linux > Linux Distro Packages section and review its current wording. Add concise paragraphs recognizing the community-maintained Flatpak, AUR, and Snap builds, while clearly stating that they are unofficial and not maintained by the project. Done means the section documents these options without implying official support.

Written by the indexing model from the issue text.