docs: Address CI docs checker warnings about heading levels

Author: euanh

Sources/swift-container-plugin/Documentation.docc/Adding-the-plugin-to-your-project.md

@@ -2,17 +2,19 @@
 
 Make Swift Container Plugin available in your project
 
+## Overview
+
 Swift Container Plugin is distributed as a Swift Package Manager package.    To make it available, you must add it as a dependency of your project.
 
-## Install the plugin using the `swift package` CLI
+### Install the plugin using the `swift package` CLI
 
 Recent versions of `swift package` suupport the `add-dependency` command:
 
 ```shell
 swift package add-dependency https://github.com/apple/swift-container-plugin --from 0.5.0
 ```
 
-## Install the plugin by manually editing `Package.swift`
+### Install the plugin by manually editing `Package.swift`
 
 If you cannot use the `swift package add-dependency` comand, append the following lines to your project's `Package.swift` file:
 
@@ -22,7 +24,7 @@ package.dependencies += [
 ]
 ```
 
-## Check that the plugin is available
+### Check that the plugin is available
 
 After installation, Swift Package Manager should show that the `ContainerImageBuilder` is now available:
 

Sources/swift-container-plugin/Documentation.docc/Platforms-and-requirements.md

@@ -2,16 +2,18 @@
 
 The open source Swift toolchain and a compatible Swift SDK make it possible to build Linux executables on macOS
 
+## Overview
+
 * Swift Container Plugin runs on macOS and Linux and requires Swift 6.0 or later.
 * On macOS you must install a cross-compilation Swift SDK, such as the [Swift Static Linux SDK](https://www.swift.org/documentation/articles/static-linux-getting-started.html), in order to build executables which can run on Linux-based cloud infrastructure.
 * The Swift Static Linux SDK requires the [open source Swift toolchain](https://www.swift.org/install/macos/) to be installed.
 * A container runtime is not required to build an image, but you will need one wherever you want to run the image.
 
-## Install the open source Swift toolchain
+### Install the open source Swift toolchain
 
 Follow the instructions at [https://www.swift.org/install/macos/](https://www.swift.org/install/macos/) to install the open source Swift toolchain.   The easiest way to do this is to use the [Swiftly swift toolchain installer](https://www.swift.org/install/macos/swiftly/).
 
-## Install a Swift SDK for cross-compilation
+### Install a Swift SDK for cross-compilation
 
 If you are running on macOS, you can use a [Swift SDK](https://github.com/apple/swift-evolution/blob/main/proposals/0387-cross-compilation-destinations.md) to cross-compile your server executable for Linux.   Either: