Document DateRules.QuarterStart and DateRules.QuarterEnd (Lean #9454)

[AlexCatarino]

Summary

• Add QuarterStart / QuarterEnd rows (no-symbol and symbol overloads, Python and C#) to the Scheduled Events Date Rules reference table.
• Add QuarterStart("SPY") and QuarterEnd("SPY") examples to the Scheduled Events Examples page, mirroring the existing Month/Week pattern.
• Documents the API added in QuantConnect/Lean#9454.

Test plan

• Render the Scheduled Events > Date Rules page and confirm four new rows appear between the Week and Year entries with the correct Python/C# signatures.
• Render the Scheduled Events > Examples page and confirm the new C# and Python QuarterStart/QuarterEnd snippets appear after the Week examples.