Create comprehensive dependency and timezone documentation

[SashankBhamidi]

The problem

Dependencies like python-dateutil and tzdata aren't explained anywhere. Users see them in requirements and don't understand why they exist, leading to "unused dependency" reports like #900.

What needs documenting

• python-dateutil: Used extensively in timezone modules (rrule, tzical, timezone handling)
• tzdata: IANA timezone data (already commented in Document tzdata dependency purpose #902 but needs broader context)
• backports.zoneinfo: Python <3.9 compatibility
• pytz: Supported timezone library, available via icalendar.use_pytz()

Nicco's specific requests from #900

From @niccokunzmann in this comment:

• Document mandatory dependencies
• Explain tzdata rationale
• Clarify pytz role and migration path
• Remove pytz/zoneinfo notice from README once properly documented
• Link from pyproject.toml to comprehensive docs
• Reference [BUG] looks like unused dependency: tzdata #900 discussion for historical context

Implementation tasks

After #822 documentation restructure:

1. Create dependency rationale page explaining each dependency's purpose
2. Remove pytz/zoneinfo notice from README (move to proper docs)
3. Link from pyproject.toml to the new documentation section
4. Reference [BUG] looks like unused dependency: tzdata #900 discussion for historical context

Related issues

• [BUG] looks like unused dependency: tzdata #900 Original tzdata confusion and broader dependency discussion
• Document tzdata dependency purpose #902 tzdata comment fix (merged)
• Restructure docs #822 Documentation restructure (wait for completion)
• Restructure documentation, following Diátaxis framework #820 Diataxis framework adoption