Add DIY certificate monitoring scripts paragraph to /docs/monitoring-options/

[compumike]

This PR adds two sentences to Monitoring Service Options which suggests that users interested in writing their own certificate monitoring script, rather than relying on a monitoring service, may wish to see the article Barebone Scripts to Check SSL Certificate Expiration which is written and hosted by me at Heii On-Call.

I sincerely hope that the example scripts which I've written and tested will be a valuable resource for users.

If you have any wording changes to suggest, please let me know. (I'm also open to the possibility of contributing the example scripts to live directly on this page, if that would be preferable. But that would be a much bigger PR, so I thought I'd keep it simple for now!)

Thank you for your time.

Here is a screenshot of the page after the addition: