Add plugin "last updated"

[MaxHillebrand]

There is an official plugin to provide a reference about the last time the markdown file was updated.
I think this is useful for Wasabi Doc because this is an open source archive of knowledge with several contributors, and it might be nice to see that it is growing and maintained.

I suggest we implement this, what do you think?

#46 #48 are also plugin, so implementation might be batched.

[MaxHillebrand]

There seems to also be a lighter version that works based on the timestamp of the last git commit. This might suit our needs just fine without an extra plugin!