Investigate a link checker GitHub action

[castrojo]

A tool like this would make sure we're on top of our outgoing links:

https://github.com/JustinBeckwith/linkinator-action

[castrojo]

Example output: https://github.com/openjs-foundation/cross-project-council/pull/728/checks?check_run_id=3624439593

[castrojo]

I wasn't able to run this on the rst source, but found busted links in the rendered outputted docs, I can PR those fixes in, here they are:

docs/aws/examples/accountinvalidiplogin.html
  [0] http://www.analyticsmarket.com/freetools/ipregex
docs/tools/c7n-mailer.html
  [404] docs/tools/c7n_mailer/cli.py
  [404] http://jinja.pocoo.org/docs/dev/templates/
docs/aws/examples/lambdaerrorsnotify.html
  [404] https://devops4solutions.com/cloud-custodian-configure-email/