Add more user-actionable information to the timezone health check

[johnbillion]

The timezone health check added in #48692 is very handy, but it doesn't provide much information for a user to action to remedy the issue.

Other health checks, such as the error display health check, provide a link to more information on the support handbook. It would be great to:

• Get a page added to the support handbook about date_default_timezone_set() and how you can go about finding and removing it from your site
• Adding a link to this page in the timezone health check

[SergeyBiryukov]

Related: #50389