remove markdown-preview plugin, re-enable Pi builds in CI

markdown-preview.nvim runs yarn install with native Node.js binaries
that crash under QEMU aarch64 emulation. The plugin is also useless
on headless servers (requires a browser). Removing it allows the Pi
build to succeed in CI again.

Re-enabled Pi build and SD image jobs in both ci.yml and deploy.yml.

modules/nixos/nixvim/plugins/default.nix

@@ -11,8 +11,6 @@
   ];
 
   config.programs.nixvim.plugins = {
-    markdown-preview.enable = lib.mkDefault true;
-    # warning: Nixvim: `plugins.web-devicons` was enabled automatically because the following plugins are enabled. This behaviour is deprecated. Please explicitly define `plugins.web-devicons.enable`
     web-devicons.enable = true;
   };
 }