Add source attribution information to images/figures

[rowanc1]

trafficstars

In many project that I have seen there is attribution information that is included for images/figures, and this is generally done ad hoc in the figure caption or under this image. We should have a way to do this in myst on the options for an image/figure. I think this could allow us to have a nicer UI around this, and also store this as structured data that we can show in a variety of ways.

https://projectpythia-mystmd.github.io/radar-cookbook/radar-basics

I am thinking something like:

:::{image} local-image.png
:source: Sundry Photography
:source_url: https://...
:source_license: Public Domain
:::

cc @agoose77