Slideshow Block: consider displaying captions below the images

When adding captions to images within a Slideshow block, the captions are displayed **above** the image:

While this may look good in some cases, it can also hide important parts of an image (the bottom of the image may display important information to the understanding of the image), or can go against the framing chosen by the photographer.

Maybe we could consider displaying those captions **below** the images instead?

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Suggested here:
https://jetpack.com/2021/01/12/jetpack-9-3-happy-new-year-from-jetpack/#comment-216325

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Of note, this also applies to captions for core galleries; it may be worth suggesting this to Core as well after our discussion here.

For reference:


No file or test is named in the issue. Start by locating the Slideshow block's caption rendering entry point and reviewing how captions are positioned relative to images; check whether core gallery captions are in scope. Done means the requested caption placement is implemented consistently and its behavior is covered by the relevant existing checks.

Written by the indexing model from the issue text.