Refactor Portal Learn MainContainer to use Structural Injection Pattern

Portal.view.learn.MainContainer extends Portal.view.shared.content.Container but still uses the old configuration pattern (defining contentComponent as a top-level config).

Since Shared.Container has been refactored to use the Structural Injection Pattern (via mergeFrom and pageContainerConfig_), Learn.MainContainer must be updated to conform to this new API.

Changes:

1. Remove contentComponent config.
2. Override pageContainerConfig to inject the ContentComponent.
3. Ensure any other inherited configs (like treeConfig) are handled if needed (default is null, which is fine here if not used, or if used it should be configured). Learn section likely uses the tree.

Current Code:

class MainContainer extends SharedContainer {
    static config = {
        // ...
        contentComponent: ContentComponent,
        // ...
    }
}

New Code:

class MainContainer extends SharedContainer {
    static config = {
        // ...
        pageContainerConfig: {
            contentConfig: {
                module: ContentComponent
            }
        },
        // ...
    }
}