docs: add build target docs-check-urls

Use the command

make docs-check-urls

to verify that all used links in the documentation are still valid.
Exceptions can be configured in docs/manuals/source/conf.py

As this command can be time consuming and error prone (due to internet or server problems),
it is not configured to run automatically.

docs/manuals/CMakeLists.txt

@@ -135,6 +135,14 @@ if(${docs-only})
   set(DOCS_ALL ALL) # this enables "make docs" by just typing "make"
 endif()
 
+add_custom_target(
+  docs-check-urls
+  COMMAND ${SPHINX_COMMAND} -M linkcheck "${SPHINX_SOURCE_DIR}" "${SPHINX_BUILDDIR}"
+  WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+  COMMENT
+    "Check if the URLs used in the documentation are still valid targets."
+)
+
 # target: docs ######
 add_custom_target(
   docs