Add a check for broken or redirecting external links

.gitlab-ci.yml

@@ -1,6 +1,25 @@
 image: registry.gitlab.com/paddy-hack/sphinx/debian/pdf:2.2.1
 
+stages:
+  - test
+  - publish
+
+test:
+  stage: test
+  before_script:
+    - apt-get update  -qq
+    - apt-get install -qqy --no-install-recommends
+              python3-setuptools
+              python3-wheel
+    - pip3 --quiet install -r requirements.txt
+  script:
+    # Prevent versioning support from kicking in as that breaks
+    # sphinx-build invocations.
+    - unset CI
+    - sphinx-build -M linkcheck . _build/
+
 pages:
+  stage: publish
   before_script:
     - apt-get update  -qq
     - apt-get install -qqy --no-install-recommends
@@ -9,7 +28,7 @@ pages:
               git
     - pip3 --quiet install -r requirements.txt
   script:
-    - sphinx-versioning build -P sane-standard.pdf ./ public/
+    - sphinx-versioning build -P sane-standard.pdf . public/
   artifacts:
     paths:
       - public

README.md

@@ -28,16 +28,18 @@ Docker image that is used in CI is another option.
 
 ## Building All Tagged Versions
 
-The CI setup for this project creates HTML and PDF versions for all
-tagged versions.  You can do so too.  In addition to everything that
-is needed to build the latest version, you will need to install the
-`sphinx-versions` Python package.  If you installed everything listed
-in [requirements.txt][RQ], you are all set.
+The CI setup for this project publishes HTML and PDF versions for all
+tagged versions.  You can create all these versions locally too.
+
+In addition to everything that is needed to build the latest version,
+you will need to install the `sphinx-versions` Python package.  If you
+installed everything listed in [requirements.txt][RQ], you are all
+set.
 
 The build itself is done with
 
 ``` sh
-CI=true sphinx-versioning build -P sane-standard.pdf ./ public/
+CI=true sphinx-versioning build -P sane-standard.pdf . public/
 ```
 
 Of course, all this can be found back in the [.gitlab-ci.yml][CI] file