GitLab

See https://github.com/cython/cython/issues/4187

* .gitignore: add directory for docs build and cython_debug
* doc-requirements.txt: add sphinx-tabs
* conf.py: add and setup sphinx-tabs extension
* Create _static\css\tabs.css for customisation
* add "two-syntax-variants-used" file as a preface about the different typing variants