alacritty: install example config
alacritty: install example config

A user recently asked where to find Alacritty's default config on NixOS
[1], since we don't have `/usr/share/doc/` (which is where other
operating systems, like Arch, install it). Now it can be accessed by
building alacritty and referring to `result/share/doc/alacritty.yml`.

[1] https://logs.nix.samueldr.com/nixos/2020-06-13#3598860;

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