This change adds `nitpicky=True` (which is an equivalent of `-n`) to make Sphinx emit warnings for any references to non-existing targets. Then, it adds `-W` to make it fail whenever a single warning is seen. Finally, `--keep-going` allows Sphinx to print out all the warnings before exiting instead of showing just one and bailing. Resolves #29 Refs: * https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-n * https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-W * https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-keep-going