gcal: add demo and flesh out documentation

gcal/README.md

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 # Gregorian Calendar
+
+![demo](demo.png)
+
+## What is this?
+
+This is an implementation of the [Gregorian calendar][0], which is the default
+civil calendar in most countries. It is meant to help you cross-reference dates
+by providing the conversions various other calendars and time-keeping systems,
+such as:
+
+1. the [Julian day number][2] (JDN);
+2. the [Julian calendar][1] date;
+3. the [Mesoamerican Long Count calendar][3] date; and
+4. the [French Republican calendar][4] date.
+
+You can [see it live here][5]! More conversions will be added in the future.
+
+## The Gregorian calendar
+
+The Gregorian calendar itself is based on the Julian calendar, originally
+introduced in 46 BC by Julius Caesar (with aid of Sosigenes of Alexandria).
+Unfortunately, the Julian calendar had leap days every four years without
+question, resulting in the average year having 365.25 days. However, the actual
+tropical year is roughly 365.2422 days, resulting in it gaining a day every 129
+years. This means that seasons drift, starting earlier and earlier in the year.
+
+In 1582, Pope Gregory XIII decided to fix this drift by reducing the number
+of leap days, motivated by a desire to keep the March equinox on March 21st,
+since that value was hardcoded in the calculation for the date of Easter. To
+achieve this, he made years divisible by 100 but not by 400 non-leap years,
+resulting in 97 leap years every 400 years. To bring the equinox back into
+alignment, October 5th to October 14th in 1582 were deleted, creating the
+Gregorian calendar we use today.
+
+This website extends the Gregorian calendar indefinitely into the past for
+reference reasons.
+
+[0]: https://en.wikipedia.org/wiki/Gregorian_calendar
+
+[1]: https://en.wikipedia.org/wiki/Julian_calendar
+
+[2]: https://en.wikipedia.org/wiki/Julian_day
+
+[3]: https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar
+
+[4]: https://en.wikipedia.org/wiki/French_Republican_calendar
+
+[5]: https://gcal.qt.ax/