docs: add AUR, install downloaded binary to /usr/local/bin, alias ser

[tippfehlr]

Hi, I added servicer to the Arch User Repository (AUR) (https://aur.archlinux.org/packages/servicer)!
This PR adds that to the README.

Also:

• The download link in the readme was outdated; I’ve set it to always point to the latest release.
• Manually putting things in /usr/bin/ is a bad idea as it’s managed by package managers.
• I’ve moved servicer to /usr/local/bin/ (the place for manually adding binaries)
• I’ve removed the symlink to ser and instead provided docs how to alias it for bash, zsh and fish.