docs(api): add configtest command

src/content/api/cli.md

@@ -46,6 +46,7 @@ webpack-cli offers a variety of commands to make working with webpack easy. By d
 | `plugin` | p | Scaffold a plugin repository |
 | `info` | i | Output information about your system and dependencies |
 | `serve` | s | Run the webpack Dev Server |
+| `configtest` | t | Tests webpack configuration against validation errors |
 | `bundle` | b | Run webpack (default command, can be omitted) |
 | `help` | h | Display help for commands and options |
 | `version` | v | Output the version of `webpack`, `webpack-cli` and `webpack-dev-server` |