Last update date

[ptbrowne]

It would be nice to have on every page a last update date to know if the article is potentially out of date. We could maybe leverage the meta attributes of the markdown.

To test:

1. Use the on_files event through a plugin to check the last update date via git and add it to the meta information of the file (https://www.mkdocs.org/user-guide/plugins/#on_files), this would not work for external docs since git information is lost when documentation files are copied from the external repository to the src repository
2. Change the template to display this information through the page.meta Jinja variable: https://www.mkdocs.org/user-guide/custom-themes/#pagemeta