changelog: Reference the opam coq repository ingestion

Well, it will be true during the day so i'll wait for it to finish. I'm using those diffs as a kind of "heads up".

Related to swh/infra/sysadm-environment#3717 (closed)

Test Plan

make -C docs happy

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Migrated from D6647 (view on Phabricator)

docs/archive-changelog.rst

@@ -13,6 +13,10 @@ in this document for historical reasons.
 2021
 ----
 
+* **2021-11-17:** Completed first archival of the `Opam Coq repository
+  <https://coq.inria.fr/opam/released/>`_. Regular cralwing of those repositories
+  enabled (tracking: `T3717 <https://forge.softwareheritage.org/T3717>`_)
+
 * **2021-10-14:** Completed archival of Bitbucket_ Mercurial repositories
   (tracking: `T3338 <https://forge.softwareheritage.org/T3338>`_)