docs: Add installation method with gah

[marverix]

Hello!
This PR adds to the documentation installation method using gah:

gah is an GitHub Releases app installer, that DOES NOT REQUIRE SUDO! It is a simple bash script that downloads the latest release of an app from GitHub and installs it in ~/.local/bin. It is designed to be used with apps that are distributed as a single binary file.

[svenstaro]

Thanks but I don't think we need it in the docs twice. Can you add a With gah section below With cargo?

[marverix]

Thanks but I don't think we need it in the docs twice. Can you add a With gah section below With cargo?

Roger that. Is now acceptable?

[svenstaro]

Cool, thanks!