Fix video alignment after dependency update without CSS

[tetrapod00]

Retry of #10566, see that PR for details of the problem we're solving.

Tested on Firefox and Edge, checked videos and tables.

This time, use :align: default for each video, but don't add any custom CSS. This will align videos on the left without floating:


Since no CSS is changed, tables still render correctly:

Left alignment matches the behavior of 4.3 videos. While I think center alignment looks a bit better, I don't see a clear way to do it without introducing additional tags at every video usage, and left alignment looks fine and matches existing behavior.

Why we can't use :align: left, as the plugin intends:
It left-aligns with float due to our Sphinx theme, see the original PR for details.

Why we can't use :align: center to center, as the plugin intends:
It left-aligns due to our Sphinx theme.

Why we can't use :align: default with custom CSS just for videos, to center:
The video plugin places the video inside a div which only has the align-default class, so there's no way to specify align-default just for the videos. We could add a class with :class:, see here. But after reverting the first fix, I'm not sure trying to maintain custom CSS just to center videos is worth the maintenance, and this would also require the use of an extra tag at every video instance.

[mhilbrunner]

Thanks!