gitlab: build HTML documentation

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

.gitlab-ci.yml

@@ -122,6 +122,13 @@ grep TODO/FIXME/HACK:
     # search for HACK within source tree and ignore HACKKIT board
     - grep -r HACK . | grep -v HACKKIT
 
+# build HTML documentation
+htmldocs:
+  tags: [ 'all' ]
+  stage: testsuites
+  script:
+    - make htmldocs
+
 # some statistics about the code base
 sloccount:
   tags: [ 'all' ]