took me a while, but i managed to figure out how to show a preview of a post instead of the whole post.

you can do it with a markdown comment as such:

<!-- TEASER_END -->

this reference by adriaanrol gave me the initial idea that i had to change the nikola conf.py file.

# Show teasers (instead of full posts) in indexes? Defaults to False.
INDEX_TEASERS = True

but it was this post by mattioo that showed me how to do it for markdown.