Add meaningful date to documentation that can evolve

In some places in the documentation one can find information that becomes quickly outdated. For example, on the page dedicated to archive copies one can read //"At the time of writing, the primary object storage contains about 5 billion blobs"// which is of course no longer true today.

Since there is no mention of what the "time of writing" is, a reader will trust the date in the footer that states //"Last updated on 2022-09-27 02:24:01 UTC."//

We need a way to avoid this kind of incoherence, and distinguish clearly the date of the regeneration of the page via Sphinx (the last updated date) from the date of creation or update of the content, which is different.

On a separate note, we should mark clearly the pages that need regular updating.

---

Migrated from T4577 (view on Phabricator)