docs: add defaultBrowser config option

docs/app/references/configuration.mdx

@@ -73,6 +73,7 @@ default values.
 | Option                 | Default                               | Description                                                                                                                                                                                                                                                                                               |
 | ---------------------- | ------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `clientCertificates`   | `[]`                                  | An optional array of [client certificates](/app/references/client-certificates).                                                                                                                                                                                                                          |
+| `defaultBrowser`       | `null`                                | The default browser to launch if the "--browser" command line option is not provided. This option only affects the first browser launched; changing this option after the browser is already launched will have no effect.                                                                                |
 | `env`                  | `{}`                                  | Any values to be set as [environment variables](/app/references/environment-variables).                                                                                                                                                                                                                   |
 | `includeShadowDom`     | `false`                               | Whether to traverse shadow DOM boundaries and include elements within the shadow DOM in the results of query commands (e.g. [`cy.get()`](/api/commands/get)).                                                                                                                                             |
 | `numTestsKeptInMemory` | Default based on `run` or `open` mode | The number of tests for which snapshots and command data are kept in memory. `numTestsKeptInMemory` is set to `50` by default during `cypress open` and set to `0` by default during `cypress run`. Reduce this number if you are experiencing high memory consumption in your browser during a test run. |