docs: Fix or remove some broken external links

Those were spotted thanks to sphinx linkcheck builder.

docs/iana-swh-template.txt

@@ -8,7 +8,7 @@ Applications/protocols that use this scheme name:
   Software Heritage: https://www.softwareheritage.org/
   Software Package Data Exchange: https://spdx.org/
   NTIA: https://www.ntia.doc.gov/SoftwareTransparency
-  Identifiers.org: http://identifiers.org/
+  Identifiers.org: https://identifiers.org/
   Name-to-Thing (N2T): https://n2t.net/
   HAL: https://hal.archives-ouvertes.fr/