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

[gregorylearns]

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 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

Thanks!