Link docs how to get zsh completion to work in getting-started

I tried to make completions work in zsh by just adding the same code as for the bash example but of course switching bash for zsh but it did not work. When I googled and dug deeper I finally found the answer in the deeper docs here:

https://github.com/fluxcd/flux2/blob/main/docs/cmd/flux_completion_zsh.md



The command in there works if I add it to my .zshrc file. I think linking to these specific docs may prevent others from just assuming it will work the same in zsh.

Signed-off-by: Jonas Kello <jonas.kello@gmail.com>