Depending on git info to generate the modified date is kinda convenient
in that I don't have to maintain the date manually, but can be
wrong (e.g., do something in git that touches the file, like moving it,
but doesn't meaningfully change it's content) and means there's a
dynamic element to building the site (generating the site depends on
information outside of the source files), which complicates CI.

So I want to move the modified date into the YAML front matter of the
content itself. Picking up the "published" date as well for good
measure. It's not currently used by pages and posts get it from the file
name, but may be utilized in the future and enables specifying the exact
time I consider a post published (if I want).