Update date range frequency from '15T' to '15min' in example data generation and fixture files

[sTomerG]

closes #10

Description

This pull request refines the frequency parameter in date range generation within the following files:

• docs/data/generate_example_data.py
• tests/fixtures/dt_index_fixtures.py

Changes include:

1. Updated the frequency argument in pd.date_range() from '15T' to '15min' for clarity and consistency.
2. Applied changes in both the example data generation script and the fixture files used for testing date ranges across various time zones (e.g., Europe/Amsterdam, America/New_York, UTC).

These adjustments enhance code readability by making the frequency parameter more explicit without affecting the functionality or output.

Testing:
All tests utilizing the modified fixtures pass as expected, confirming that the change does not introduce any regressions.

[leonoverweel]

Thx!