Enforce the use of fig.alt in gitbook?

[ThierryO]

I found that bookdown::gitbook has a fig.alt chunk option.
This allows to set the alt tag.
Which is useful for visually impaired readers.

We could detect at runtime if fig.alt is set. And issue a warning or error when it is missing. E.g.
https://github.com/jumpingrivers/hugo-rmd/blob/736d5e91e1c75aed245cb574d82dd7ccb15234b1/R/hugo.R#L73