azure: build HTML documentation

Several patches delivered incorrect restructured text as documentation. We
should be able to discover this in Azure CI.

Provide a build step for 'make htmldocs'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

.azure-pipelines.yml

@@ -54,6 +54,16 @@ jobs:
     steps:
       - script: cppcheck --force --quiet --inline-suppr .
 
+  - job: htmldocs
+    displayName: 'Build HTML documentation'
+    pool:
+      vmImage: $(ubuntu_vm)
+    container:
+      image: $(ci_runner_image)
+      options: $(container_option)
+    steps:
+      - script: make htmldocs
+
   - job: todo
     displayName: 'Search for TODO within source tree'
     pool: