Bibliography: Add Modified Date to YAML description

[stumbo]

Describe the bug
The front matter, the yaml header in bibliography markdown files, does not contain the last modified date of the entry in Zotero. We use modified date to seed the sitemap.xml lastmod field. Adding this value will ensure that the site map accurately reflects when pages were last modified.

Currently, when the sitemap is built the lastmod date defaults to the publication date of the bibliographic item.

Expected behavior
sitemap.xml accurately reflects the modified date of entries in Zotero instead of the default date.

Screenshots
The sitemap with lastmod:

The source Zotero entry's Modified Date: