Add package manager instructions if available on Linux

[nathanfranke]

Operating system, Browser, Browser Version:
Linux 5.18.11-arch1-1
Mozilla Firefox 102.0.1

Issue description:

Package manager commands should be included as Godot can be updated "automatically" with the rest of the system.
I suggest categories for each popular distro.

Ubuntu:
sudo snap install godot --classic (is there a better package manager to use?)

Arch-based:
sudo pacman -S godot

Any distro with flatpak:
flatpak install org.godotengine.Godot

Not sure what other commands work.

Screenshots (greatly appreciated if it’s a style issue)

Already similar instructions for Windows:

[Calinou]

Ubuntu:
sudo snap install godot --classic (is there a better package manager to use?)

Other than installing Flatpak, I'm afraid not. Distribution packages are usually outdated (and sometimes contain different bugs as a result of using system libraries), so we don't recommend using them.

We also don't recommend the Flatpak yet as it lacks full support for exporting to Android, along with C# support.