Adds an OS Compatibility section to the README and the Installation
manual page: developed and tested on Arch Linux, macOS and Windows
unsupported, with the specific Linux-only calls that back that claim
(systemd-inhibit, zramctl/swapon, sbctl, wl-copy/xclip with no
pbcopy/clip.exe fallback, GNU-only stat/numfmt flags) and the baseline
packages (git, gpg, tar, coreutils) assumed present rather than tracked
by fish-deps.
Adds os/os-compatibility/operating-system/compatibility/linux/macos/
mac/windows/wsl keywords to fish-config.index so `help config os` and
friends jump straight to it.
rootiest