Workaround for lead paragraphs: Hide the rest with CSS+JS

[OndraZizka]

Workaround until jbake-org/jbake#503

This will also need adding the section to the documents.

New cool feature!
##############

/---section .(lead)
JBake has a new feature.
\---

Coming in next release.

Maybe the JS could do it automatically?
Choose the first heading plus first few elements until the text reaches some size?
Some CSS is here: https://stackoverflow.com/questions/29598625/hide-all-except-first-child-of-an-element-using-pure-css