Add changed date of markdown files during build, remove from source

[renestalder]

As proposed by @rudigier and @AndreasUe if I'm not mistaken, the "Last updated" date, which is part of the markdown files frontmatter properties, included since the first release and now also the foundation to show the date in the footer of the page, is currently a manual field. This has to be updated by the author making changes to the content. If not updated, the date is incorrect.

Rather than making this a manual task, the field could be removed from the source markdown files and added during build time, based on the last change from the git history. Or maybe even directly from the change during the build, that might be error prone. The author wouldn't have to think about it anymore.