Add Travis CI job to build the documentation

The job will fail if there are any warnings, therefore keeping us
informed if the actual online version of the documentation would be
affected by any changes.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

.travis.yml

@@ -66,6 +66,16 @@ matrix:
         - git diff-index --quiet HEAD || git commit -m 'doxygen commit'
         - git push
 
+    # Build documentation similarly to how RTD does
+    - name: documentation
+      python: "3.8"
+      before_install:
+      install:
+        - pip install sphinx sphinx-rtd-theme
+      script:
+        - make -C docs html SPHINXOPTS="-W --keep-going"
+
+
 # We want to use docker during the tests
 services:
  - docker