CI: quiet the docs linkcheck

[rkingsbury]

Part of our continuous integration tests is an automatic check of all hyperlinks in our docs and docstrings. This is great in principle; however it seems to fail frequently for reasons beyond our control (notably, timeouts and -rate-limited cases. When it fails, it's also difficult to identify where the failure came from without reading every line of the output.

We need to find a way to make the output of this test more useful and to prevent it from failing for reasons beyond our control. I don't know much about the configuration options, but ideally, we should treat timeouts and -rate-limited cases as warnings not errors (in other words, ignore those issues).