Add docs/WIKI.md and enterprise _example/ programs for timefy

[Copilot]

Adds a professor-level technical wiki and five runnable enterprise example programs covering the full timefy API surface from first principles to production patterns.

docs/WIKI.md (2,220 lines)

• 27 sections: installation → core types → every boundary/parse/format/comparison API → advanced patterns → enterprise use cases → full quick-reference tables
• Each section explains the why behind design decisions (e.g., why EndOfDay returns 23:59:59.999999999, why EndOfMonth delegates to AddDate(0,1,0).Add(-1ns), when to use SetTimezone vs AdjustTimezone)
• Precise documentation of GetWeekdaysInRange leap-year limitation with a portable IsWeekday-based workaround
• Format constant table, timezone constant table, and complete method-level API reference

_example/ — Five enterprise programs

Program	Scenario	Highlighted APIs
billing/	SaaS pro-rate, trial expiry, timezone-aware month boundaries	BeginningOfMonth, EndOfQuarter, IsBetween, AdjustTimezone
global_team/	5-city overlap windows, public holidays, world clock	AddHour, AddDay, IsWeekday, IsSameDay
audit/	GDPR 72h compliance, SLA breach, hourly event bucketing	AddHour, BeginningOfHour, IsBetween, FormatTimex
reporting/	SEC 10-Q/10-K windows, YTD, QoQ, July-start fiscal year	BeginningOfQuarter, EndOfYear, DurationInDays
scheduler/	Business-day ETL jobs, anti-duplication window	BeginningOfQuarter, AddMinute, IsWeekday

.gitignore

Added !_example/** exception — the existing main.go rule was silently excluding all example entry points.

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