add(elements): Template for defining an Element

Elements.md

@@ -8,6 +8,39 @@ library should instead provide small examples to show how you can implement
 such user fields yourself and connect them using your own event system loops to
 communicate with other `Container`s and/or `Element`s.
 
+## Template
+
+```zig
+/// This is an empty template implementation for an `Element` type.
+pub fn Template(Event: type) type {
+    return packed struct {
+        pub fn element(this: *@This()) Element(Event) {
+            return .{
+                .ptr = this,
+                .vtable = &.{
+                    .handle = handle,
+                    .content = content,
+                },
+            };
+        }
+
+        fn handle(ctx: *anyopaque, event: Event) !void {
+            const this: *@This() = @ptrCast(@alignCast(ctx));
+            _ = this;
+            switch (event) {
+                else => {},
+            }
+        }
+
+        fn content(ctx: *anyopaque, cells: []Cell, size: Size) !void {
+            const this: *@This() = @ptrCast(@alignCast(ctx));
+            std.debug.assert(cells.len == @as(usize, size.cols) * @as(usize, size.rows));
+            _ = this;
+        }
+    };
+}
+```
+
 ### User specific event handling and content rendering
 
 For interactions controlled by the user each container can use an `Element`