Add a section in the gallery page for recent notebooks added

[Dekermanjian]

This is not a notebook proposal. It just did not really fit in the other categories. I am proposing that on the website in the gallery area (https://www.pymc.io/projects/examples/en/latest/gallery.html) to include at the top a section for recently added notebooks. I think this would be a nice addition to folks that visit the website often.