DOC: replace deprecated US/Eastern timezone alias with America/New_York

[dgowdaan-cmyk]

Partial fix for #2795.

Replaces the deprecated "link" type timezone alias US/Eastern with
its canonical name America/New_York in
docs/examples/solar-tracking/plot_discontinuous_tracking.py, which was
failing the RTD documentation build.

This PR intentionally covers only this one file to keep the change
small and easy to review. Several other files in docs/examples,
docs/tutorials, and docs/sphinx also use deprecated aliases
(Asia/Calcutta, US/Mountain, US/Arizona, MST) — happy to open follow-up
PRs for those once the approach here is confirmed, especially since a
couple of the affected locations (Golden, CO) need careful checking
to use the geographically correct canonical zone (America/Denver,
not America/Phoenix).

• Closes (partial, see note above)
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes
• Adds whatsnew entry

[github-actions]

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