[cleanup] Fix Contributing doc TOC and list numbers (flutter#8105)

Fixes up stale TOC and a step mis-numbering.

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](flutter#8098)).
</details>

Author: pq

CONTRIBUTING.md

@@ -5,15 +5,21 @@ Contributing to Flutter Plugin for IntelliJ
 * [Contributing to Flutter Plugin for IntelliJ](#contributing-to-flutter-plugin-for-intellij)
   * [Contributing code](#contributing-code)
   * [Getting started](#getting-started)
-  * [Setting environments](#setting-environments)
+  * [Environment set-up](#environment-set-up)
+  * [IntelliJ set-up](#intellij-set-up)
+    * [Configure "Project Structure" settings](#configure-project-structure-settings)
+    * [Configure the Gradle settings](#configure-the-gradle-settings)
+    * [Build and run the plugin](#build-and-run-the-plugin)
     * [Handle symlinks](#handle-symlinks)
   * [Provision Tool](#provision-tool)
   * [Running plugin tests](#running-plugin-tests)
     * [Using test run configurations in IntelliJ](#using-test-run-configurations-in-intellij)
     * [Using the plugin tool on the command line](#using-the-plugin-tool-on-the-command-line)
   * [Adding platform sources](#adding-platform-sources)
   * [Working with Android Studio](#working-with-android-studio)
-  * [Working with Embedded DevTools (JxBrowser)](#working-with-embedded-devtools-jxbrowser)
+  * [Flutter DevTools Integration](#flutter-devtools-integration)
+    * [Working with Embedded DevTools (JxBrowser)](#working-with-embedded-devtools-jxbrowser)
+    * [Developing with local DevTools](#developing-with-local-devtools)
 <!-- TOC -->
 
 ## Contributing code
@@ -176,21 +182,21 @@ name and contact info to the [AUTHORS](AUTHORS) file.
     Internal Java Platform: IDEA JBR 21
     ``` 
 
-3. Set the SDK for the Project
+4. Set the SDK for the Project
     - Select "Project Settings > Project"
     - Change the "SDK" selection to the **IntelliJ Platform Plugin SDK** you added in step 3.
     - When you are done, your settings should look something like:
     ```
     SDK: IntelliJ IDEA Community Edition IC-243.23654.189
     ```
 
-4. Configure the modules for the Project
+5. Configure the modules for the Project
     - Select "Project Settings > Modules"
     - Select the `flutter-intellij > flutter-idea` module
     - Switch to the "Paths" window
     - Select the **Inherit project compile output path** option then apply.
 
-5. Change the modules SDK to the Project SDK
+6. Change the modules SDK to the Project SDK
     - Select "Project Settings > Modules"
     - Select all the sub-directories under the `flutter-intellij > flutter-idea` module
     - Switch to the "Dependencies" window