docs: added docs for CRON_TZ

[jonnochoo]

I have added docs to highlight the support for time zone expressions

[yohamta]

LGTM🚀🚀🚀, thank you very much for improving the document!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.80%. Comparing base (6ce3fdd) to head (bc0be6b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #716   +/-   ##
=======================================
  Coverage   61.80%   61.80%           
=======================================
  Files          53       53           
  Lines        5249     5249           
=======================================
  Hits         3244     3244           
  Misses       1751     1751           
  Partials      254      254           

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