Add basic auto-completion to Neovim and comprehensive documentation

- Add nvim-cmp plugin with buffer and path completion sources
- Configure intuitive keybindings for completion navigation:
  * Ctrl+Space to trigger completion manually
  * Tab/Shift+Tab for menu navigation
  * Enter to accept completion, Ctrl+e to close menu
- Enable autoEnableSources for automatic plugin management

- Add comprehensive "Code Editing & Auto-completion" section to README:
  * Document all key features: Gruvbox theme, Tree-sitter, file explorer
  * Provide complete auto-completion usage guide with keybindings
  * Include essential keybinding reference for file operations, clipboard, git
  * Explain completion sources: buffer words and file path suggestions

Benefits:
- Simple, non-intrusive auto-completion without language servers
- Word completion from open buffers for faster typing
- Path completion for file navigation and imports
- Clear documentation for all Neovim features and keybindings
- Follows standard completion UI patterns users expect

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

modules/nvim.nix

@@ -86,6 +86,27 @@
       render-markdown = {
         enable = true;
       };
+
+      # Basic auto-completion
+      cmp = {
+        enable = true;
+        autoEnableSources = true;
+        
+        settings = {
+          sources = [
+            { name = "buffer"; }     # Words from open buffers
+            { name = "path"; }       # File system paths
+          ];
+          
+          mapping = {
+            "<C-Space>" = "cmp.mapping.complete()";          # Trigger completion manually
+            "<C-e>" = "cmp.mapping.close()";                 # Close completion menu
+            "<CR>" = "cmp.mapping.confirm({ select = true })"; # Accept selected completion
+            "<Tab>" = "cmp.mapping.select_next_item()";      # Navigate down in menu
+            "<S-Tab>" = "cmp.mapping.select_prev_item()";    # Navigate up in menu
+          };
+        };
+      };
     };
 
     # Extra plugins that don't have dedicated modules