Last update date

It would be nice to have on every page a last update date to know if the article is potentially out of date.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

Start with the MkDocs on_files plugin event and the custom-theme page.meta Jinja variable described in the issue. Determine how the last update date can be added from git metadata, then verify that the custom theme displays it on every page while accounting for external documentation without git history.