Revisit references to 3P websites/apis

We should minimize references to 3P websites/apis unless they have a well-known owner and is stable enough, such as npm or github.

For example, this document references [crontab.guru](https://crontab.guru/) just for getting the correct cron syntax. Can we just write the correct rules in place?

https://github.com/microsoft/GitHub-Copilot-for-Azure/blob/d01826fce9dc4139dc52230338a1808ab74bbc86/plugin/skills/azure-prepare/references/services/functions/templates/recipes/timer/README.md?plain=1#L65

Referencing 3P websites/apis make agents using the skills more subject to external threats since each 3P website/api may become a source of attack if it gets compromised. In addition, 3P websites and apis are subject to external change, which means we will need to keep monitoring their changes and update our skills accordingly.