doc: Add an `@introducedin` tag.

It is optional for now, but maybe in the future we can make
it mandatory. It will allow to have some "New!", "Not released"
messages or something in the doc. That would allow us to share
most of the doc between the stable and git versions.

docs/config.ld

@@ -377,6 +377,16 @@ add_custom_tag {
     },
 }
 
+-- Specify when an item was added.
+-- @introducedin 4.4
+add_custom_tag {
+    name   = "introducedin",
+    hidden = true,
+    params = {
+        { name = "api_level" },
+    },
+}
+
 -- More fitting section names
 kind_names={topic='Documentation', module='Libraries', script='Sample files'}