Regularly check for broken links

Inspired by https://github.com/dask/dask-examples/pull/151, which I encountered manually, I just ran http://examples.dask.org through a dead link checker and found a handful of broken links. I don't have time to fix them all right now but I just thought I'd drop the results here.

It might be a good idea to include a dead link check as part of the website deploy, but that may also be overkill!

| Status | URL | Source link text |
|-----------|-------|--------|
| -1 Invalid URL | | [http://127.0.0.1:8787/status](https://examples.dask.org/bag.html "http://127.0.0.1:8787/status") |
| 404 Not Found | | [Dask Bag Documentation](https://examples.dask.org/bag.html "Dask Bag Documentation") |
| 404 Not Found | | [](https://examples.dask.org/machine-learning/incremental.html) |
| -1 Invalid URL | | [http://10.20.0.141:8787/status](https://examples.dask.org/array.html "http://10.20.0.141:8787/status") |
| 404 Not Found | | [http://ml.dask.org/examples/xgboost.html](https://examples.dask.org/machine-learning/xgboost.html "http://ml.dask.org/examples/xgboost.html") |
| 404 Not Found | | [https://xgboost.readthedocs.io/en/latest/python/python_intro](https://examples.dask.org/machine-learning/xgboost.html "https://xgboost.readthedocs.io/en/latest/python/python_intro") |
| 404 Not Found | | [local cluster](https://examples.dask.org/machine-learning/voting-classifier.html "local cluster") |
| 404 Not Found | | [https://docs.scipy.org/doc/numpy-1.16.0/reference/c-api](https://examples.dask.org/applications/stencils-with-numba.html "https://docs.scipy.org/doc/numpy-1.16.0/reference/c-api") |
| 404 Not Found | | [user guide [301 from http://scikit-learn.org/stable/modules/scaling_strategies.html]](https://examples.dask.org/machine-learning/incremental.html "user guide [301 from http://scikit-learn.org/stable/modules/scaling_strategies.html]") |
| 404 Not Found | | [Generalized Universal Functions [302 from https://docs.scipy.org/doc/numpy/reference/c-api.generalized-ufuncs.html]](https://examples.dask.org/applications/stencils-with-numba.html "Generalized Universal Functions [302 from https://docs.scipy.org/doc/numpy/reference/c-api.generalized-ufuncs.html]") |
| 404 Not Found | | [dashboard's status page](https://examples.dask.org/delayed.html "dashboard's status page") |
| 404 Not Found | | [dashboard's graph page](https://examples.dask.org/delayed.html "dashboard's graph page") |
| 404 Not Found | [https://examples.dask.org/applications/'](https://examples.dask.org/applications/ "source URL: https://examples.dask.org/applications/image-processing.html") | [Cleaning up temporary directories and files](https://examples.dask.org/applications/image-processing.html "Cleaning up temporary directories and files") |
| 404 Not Found | | [img/src](https://examples.dask.org/applications/prefect-etl.html "img/src") |
| 404 Not Found | | [dask examples](https://examples.dask.org/surveys/2019.html "dask examples") |
| -1 Timeout | | [Celery](https://examples.dask.org/applications/async-web-server.html "Celery") |
| 404 Not Found | | [scale out to a cluster](https://examples.dask.org/applications/async-web-server.html "scale out to a cluster") |