[IMP] Add image option to cards, update guidelines

[nikibrown]

Summary:

Extends the custom Card RST directive to allow for images and alt text based on bootstrap conventions. Added an example in the guidelines and tweaked layout javascript to make card images render correctly.

Task: https://www.odoo.com/odoo/project/3835/tasks/6393116

 .. card:: Card with image
                :target: ../documentation
                :large:
                :image: placeholder-card-image.png
                :alt: Placeholder card image

                Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed do eiusmod tempor.

This 17.0 PR can be FWP up to master.

[robodoo]

[AntoineVDV]

Hi, the diff LGTM from a technical perspective and it doesn't cost much to run and maintain, even if we end up not using images in cards.

[AntoineVDV]

6KB for such a simple image is suspiciously heavy; it can most probably be reduced.