doc: add working offline tutorial to examples gallery (#956)

[KomalDeep355]

Fixes #956

Summary

Adds a new tutorial 'examples/30_extended/offline_tutorial.py'
demonstrating how to use OpenML-Python without an active internet connection
connection.

What's covered

• Populating the local cache using 'populate_cache()'
• Checking the cache directory
• Loading datasets and tasks from cache offline
• Running a basic ML experiment using only cached data

Notes

Follows the existing gallery format used in 'examples/30_extended/'

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.53%. Comparing base (54e94a3) to head (13f609a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1725   +/-   ##
=======================================
  Coverage   53.53%   53.53%           
=======================================
  Files          63       63           
  Lines        5234     5234           
=======================================
  Hits         2802     2802           
  Misses       2432     2432           

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