Add documentation for the time zone configurations

[yohamta]

This PR Adds the documentation about tz configuration via config file and environment variables. Also includes some refactoring regarding time zone handling.

• Added the documentation for tz
• Refactored the scheduler to use the specified timezone setting
• Refactored the environment config key to DAGU_TZ

[codecov]

Codecov Report

Attention: Patch coverage is 28.20513% with 28 lines in your changes missing coverage. Please review.

Project coverage is 61.97%. Comparing base (93c3710) to head (e6fb91b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/config/config.go	0.00%	17 Missing ⚠️
internal/scheduler/scheduler.go	35.29%	10 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
- Coverage   62.10%   61.97%   -0.14%     
==========================================
  Files          53       53              
  Lines        5215     5228      +13     
==========================================
+ Hits         3239     3240       +1     
- Misses       1723     1734      +11     
- Partials      253      254       +1     

Files with missing lines	Coverage Δ
internal/scheduler/entryreader.go	67.82% <100.00%> (ø)
internal/scheduler/scheduler.go	64.49% <35.29%> (+2.08%)	⬆️
internal/config/config.go	0.00% <0.00%> (ø)

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