Added an overview of the containers subtypes

[Ymanawat]

This PR resolves issue #7715 and includes the necessary changes for all the branches that have these containers (maybe 3.2+).

It introduces an overview of three container subtypes, namely AspectRatioContainer, FlowContainer, and CenterContainer. These subtypes were missing from the existing documentation at https://docs.godotengine.org/en/stable/tutorials/ui/gui_containers.html.

• Bugsquad edit, fixes: Mutliple Container Subtypes are not documented under the Container Overview reference in the Manual #7715

[paddy-exe]

Hey @Ymanawat Thanks for your PR! Could you please convert the images you added to .webp following the docs contribution guidelines: https://docs.godotengine.org/en/stable/contributing/documentation/docs_image_guidelines.html?

[Ymanawat]

Hello @paddy-exe! I've converted all the images to .webp

[DarthKitten2130]

Thanks for the PR! (I opened the issue)

[mhilbrunner]

Thanks, and thanks for converting the images to WebP 🙏 Merged. Amazing first contribution!