CI: detect errors in `make doc`
Right now errors in process of generating docs are not detected in CI. At least very basic detection would help to catch typos early on. Example:
/home/pspacek/pkg/kresd/git/doc/modules.rst:28: SEVERE: Problems with "include" directive path: InputError: [Errno 2] No such file or directory: '../modules/ta-signal/README.rst'.