Summaries

[jplatte]

Zola has support for generating a summary of a page based on a <!-- more --> marker in the content, to display it in overviews. Used on https://matrix.org/blog/ for example. If #6 is implemented the way I'm thinking right now, this is complicated further as a page listing subdirectory pages wants metadata it gets to include summaries. Maybe there are even use cases where people want to render a snippet of the previous and next pages, but I think I'd rather just not support that.

Implementation note: PageMetadata could just contain an optional field for the summary which is set to None by the functions that get the previous and next page.