Cookbook example of custom calendar

Add a page showing how to do a custom calendar by creating a class that
works like PlainDate, but implements a calendar that isn't built in to
JavaScript.

Author: ptomato

docs/cookbook-nepali-calendar.md

@@ -0,0 +1,11 @@
+# Nepali calendar
+
+Support for the Nepali calendar is currently not built in to JS
+environments.
+This example shows how to implement a custom calendar with Temporal, by
+creating a date class for that calendar that works like PlainDate and
+can be used as a PlainDate property bag.
+
+```javascript
+{{cookbook/nepaliCalendar.mjs}}
+```

docs/cookbook.md

@@ -567,3 +567,11 @@ Extend `Temporal` to support arbitrarily-large years (e.g., **+635427810-02-02**
 Extend `Temporal` to support adjustment days for the Hijri calendars, which are sometimes required when the start of the month is based on astronomical observations.
 
 → [Adjustable Hijri calendar](hijri-days-adjustments.md)
+
+### Nepali calendar
+
+Implementation of the Nepali calendar in userspace.
+This calendar is currently not built in to JS environments.
+This serves as an example of how to build support for calendars that are not widespread in browsers.
+
+→ [Nepali calendar](cookbook-nepali-calendar.md)

docs/cookbook/all.mjs

@@ -24,6 +24,7 @@ import './getUtcOffsetStringAtInstant.mjs';
 import './getWeeklyDaysInMonth.mjs';
 import './localTimeForFutureEvents.mjs';
 import './makeExpandedTemporal.mjs';
+import './nepaliCalendar.mjs';
 import './nextWeeklyOccurrence.mjs';
 import './noonOnDate.mjs';
 import './plusAndRoundToMonthStart.mjs';