ci: Check for broken links when build HTML manual

.gitlab/ci/configure_sphinx.cmake

@@ -4,3 +4,6 @@ set(SPHINX_HTML ON CACHE BOOL "")
 set(SPHINX_SINGLEHTML ON CACHE BOOL "")
 set(SPHINX_QTHELP ON CACHE BOOL "")
 set(SPHINX_TEXT ON CACHE BOOL "")
+if(NOT "$ENV{CMAKE_CI_NIGHTLY}" STREQUAL "")
+  set(SPHINX_LINKCHECK ON CACHE BOOL "")
+endif()