docs(printing): reduce default drivers

docs/printing.md

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+# Printing
+
+This repo enables the CUPS printing service via `modules/system/printer.nix`, but it does **not** bundle any
+printer driver packages by default.
+
+## Opt into drivers from a host file
+
+Add driver packages to `services.printing.drivers` in your host module. Example:
+
+```nix
+# hosts/my-host.nix
+{ pkgs, ... }:
+
+{
+  services.printing.drivers = [
+    pkgs.gutenprint
+  ];
+}
+```
+
+## Driver packages (cheat sheet)
+
+Generic / common:
+
+- `pkgs.gutenprint` — various printer drivers
+- `pkgs.gutenprintBin` — various binary printer drivers
+- `pkgs.cups-filters`
+- `pkgs.cups-browsed`
+
+Brother:
+
+- `pkgs.brgenml1cupswrapper`
+- `pkgs.brgenml1lpr`
+- `pkgs.brlaser`
+
+Canon:
+
+- `pkgs.cnijfilter2` — Canon Pixma drivers
+
+Epson:
+
+- `pkgs.epson-escpr`
+- `pkgs.epson-escpr2`
+
+HP:
+
+- `pkgs.hplip`
+- `pkgs.hplipWithPlugin` — proprietary plug-ins
+
+Lexmark:
+
+- `pkgs.postscript-lexmark`
+
+Samsung:
+
+- `pkgs.samsung-unified-linux-driver`
+- `pkgs.splix` — drivers for Samsung Printer Language printers