Add a link to Flathub on the Linux download page

[Calinou]

In the same vein as https://github.com/godotengine/godot-website/issues/58, it would also be useful to mention Godot's presence on Flathub for Flatpak users on the Linux download page.

[Zerokami]

Flatpak sandboxes your app and I assume there is also signature or hash verification going on with Flatpak installs, which is good.

[Juankz]

Is flatpak version officially mantained?

[Calinou]

@Juankz I'm one of its maintainers now, so I guess it can be considered official :slightly_smiling_face:

However, it is currently impossible to export to Android using the Godot Flatpak, so that should be noted on the download page somehow. (To fix this, Godot could be modified to use flatpak-spawn to start those programs directly from the host.)