Periodically check for broken links

[scottyhq]

Could be good to add a workflow to check from broken links once a month or so

from #246

(intermediate/cmip6-cloud: line 40009) broken    https://esgf-node.llnl.gov/search/cmip6/ - HTTPSConnectionPool(host='esgf-node.llnl.gov', port=443): Max retries exceeded with url: /search/cmip6/ (Caused by ConnectTimeoutError(<urllib3.connection.HTTPSConnection object at 0x7f4fd7536250>, 'Connection to esgf-node.llnl.gov timed out. (connect timeout=None)'))
(intermediate/cmip6-cloud: line 70012) broken    https://esgf-node.llnl.gov/search/cmip6 - HTTPSConnectionPool(host='esgf-node.llnl.gov', port=443): Max retries exceeded with url: /search/cmip6 (Caused by ConnectTimeoutError(<urllib3.connection.HTTPSConnection object at 0x7f4fd7500f90>, 'Connection to esgf-node.llnl.gov timed out. (connect timeout=None)'))