Many people use mkdocs for their documentation needs. Implementing a working lightbox for images does seem to be hard for some people to do! Getting a working lightbox js extension for all images in your Mardown docs is actually fairly simple, with the lightgallery-markdown package on Github.
This article offers a sample of basic Markdown syntax that can be used in Hugo content files, also it shows whether basic HTML elements are decorated with CSS in a Hugo theme.
The following HTML
<h6> elements represent six levels of section headings.
<h1> is the highest section level while
<h6> is the lowest.