nixvim: add diffview, gitsigns, which-key. re-enable treesitter. document some keymaps

modules/home/nixvim/plugins/default.nix

@@ -3,14 +3,17 @@
 {
   imports = [
     ./cmp.nix
+    ./diffview.nix
     ./lsp.nix
+    ./gitsigns.nix
     ./lualine.nix
     ./telescope.nix
-    # ./treesitter.nix # HOTFIX: does not build
+    ./treesitter.nix
     ./trouble.nix
   ];
 
   config.programs.nixvim.plugins = {
+    which-key.enable = lib.mkDefault true;
     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;