Tag guides as "new" or "updated"

[ChristopheDujarric]

Related to #215. I don't know what the limitations would be, but it would be great to have a view of "what's new" in the guides and potentially "what's been updated lately."

We're starting to have a decent amount of guides, and for our returning readers, it would be great to know quickly what is the freshest content by:

• seeing a "New" tag for a guide that has been initially published in the last 4 weeks
• (if possible, although less important) seeing an "Updated" tag for a guide which has been updated in the last 4 weeks. Worst case scenario, we can fallback on updating the prefix in the filename only for major updates of guides, and consider them as "New".
• having a list view of all "New" guides