Replace OPeNDAP datasets with Xarray tutorial datasets in docs

[tomvothecoder]

Description

• Closes [Doc]: are there some xcdat test files (that can be predownloaded) ? #277

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 99.67%. Comparing base (e3dda64) to head (b32f462).

Files with missing lines	Patch %	Lines
xcdat/tutorial.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #705      +/-   ##
===========================================
- Coverage   100.00%   99.67%   -0.33%     
===========================================
  Files           15       16       +1     
  Lines         1546     1559      +13     
===========================================
+ Hits          1546     1554       +8     
- Misses           0        5       +5     

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