Add table with calendar descriptions

Temporal moves the "Calendar types" section from ECMA-402 to ECMA-262.
This adds a new "Calendar types" section to ECMA-402, that refers to the
ECMA-262 one, and contains additional information for implementations
that conform to ECMA-402.

In particular, this adds a table with high-level descriptions of each
supported calendar system, 1 to 2 sentences long, so that
implementations can align on which calendar they refer to exactly.

"islamic" and "islamic-rgsa" are included, pending #32.

Closes: #2

Author: ptomato

biblio.json

@@ -2,6 +2,11 @@
     {
         "location": "https://tc39.es/proposal-temporal/",
         "entries": [
+            {
+                "type": "clause",
+                "number": "Temporal, 12.1",
+                "id": "sec-availablecalendars"
+            },
             {
                 "type": "clause",
                 "number": "Calendar Date Records",
@@ -27,11 +32,16 @@
                 "term": "calendar type",
                 "id": "sec-calendar-types"
             },
+            {
+                "type": "term",
+                "term": "calendar types",
+                "id": "sec-calendar-types"
+            },
             {
                 "type": "term",
                 "term": "Year-Week Record",
                 "id": "sec-year-week-record-specification-type"
             }
         ]
     }
-]
+]