Add linkcheck job

(cherry picked from commit 48eab76427)

.gitlab-ci.yml

@@ -679,6 +679,18 @@ changelog:
   artifacts:
     untracked: true
 
+linkcheck:
+  <<: *base_image
+  stage: docs
+  script:
+    - pushd doc/arm/ > /dev/null && sphinx-build -b linkcheck . linkcheck_output/
+  artifacts:
+    paths:
+      - doc/arm/linkcheck_output/
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "schedule"'
+  needs: []
+
 docs:
   <<: *default_triggering_rules
   <<: *base_image

doc/arm/conf.py

@@ -214,6 +214,20 @@ latex_documents = [
 
 latex_logo = "isc-logo.pdf"
 
+# -- Options for linkcheck ----------------------------------------------
+linkcheck_timeout = 10
+linkcheck_ignore = [
+    "http://127.0.0.1",
+    "https://gitlab.isc.org",
+    "https://kb.isc.org",
+    "https://simpleicon.com/",
+    "https://www.dnssec-or-not.com/",
+    "https://www.flaticon.com/",
+    "https://www.freepik.com/",
+    "https://www.godaddy.com",
+    "https://www.icann.org",
+]
+
 #
 # The rst_epilog will be completely overwritten from the Makefile,
 # the definition here is provided purely for situations when