Render fig-caps as visible html

[damonbayer]

#819 (comment)

In our tutorials, the quarto fig-caps are rendered as alt-text in html, not as visible captions.
Having alt text is good, but we want everyone to see the captions.

I see two options

1. Remove all fig-caps, replacing them with paragraphs below the figures.
2. Still write fig-caps in quarto, but automate duplicating / transferring them to below figure paragraphs with a quarto post-processing script.

I have implemented 2 (with the duplication option) in #825.