feat(basic-section): container_class for the image config

[guillaume-flambard]

Fixes #5830.

The image config of a basic section could only forward attributes to the img element, so applying the hide utilities left the container's reserved space on the page. Downstream sites (canonical.com/juju, ubuntu.com/ceph/install, ubuntu.com/certified) currently work around it with the deprecated CTA section slots or by breaking out of the framework entirely, as listed in the issue.

This adds a container_class key to the image config: a free-form string of utility classes appended to the p-image-container classes, alongside the existing is-highlighted and is-cover booleans. Intended use is the hide utilities, for example:

{
  "type": "image",
  "item": {
    "attrs": {"src": "hero.png", "alt": ""},
    "container_class": "u-hide--small u-hide--medium"
  }
}

Docs updated (config block + key description, pointing at the hide utilities). Renders verified for the default, the new key, and the combined aspect-ratio / no-highlight / container_class cases:

<div class="p-image-container--16-9 u-hide">
  <img class="p-image-container__image " src="a.png" alt="a" />
</div>

Happy to adjust the key name if you would rather see a different shape (a container_attrs dictionary, for instance).

[webteam-app]

guillaume-flambard is not a collaborator of the repo