feat(examples): add hello_world, update r/demo/event (#3130)

<!-- please provide a detailed description of the changes made in this
pull request. -->

## Description

We don't have a clean & simple hello_world realm. I also updated the doc
on the `r/demo/events` realm, and also renamed it to emit.

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [x] Provided any useful hints for running manual tests
</details>

examples/gno.land/r/demo/emit/emit.gno

@@ -0,0 +1,12 @@
+// Package emit demonstrates how to use the std.Emit() function
+// to emit Gno events that can be used to track data changes off-chain.
+// std.Emit is variadic; apart from the event name, it can take in any number of key-value pairs to emit.
+package emit
+
+import (
+	"std"
+)
+
+func Emit(value string) {
+	std.Emit("EventName", "key", value)
+}

examples/gno.land/r/demo/emit/gno.mod

@@ -0,0 +1 @@
+module gno.land/r/demo/emit

examples/gno.land/r/demo/event/z1_filetest.gno → examples/gno.land/r/demo/emit/z1_filetest.gno

@@ -1,34 +1,34 @@
 package main
 
-import "gno.land/r/demo/event"
+import "gno.land/r/demo/emit"
 
 func main() {
-	event.Emit("foo")
-	event.Emit("bar")
+	emit.Emit("foo")
+	emit.Emit("bar")
 }
 
 // Events:
 // [
 //   {
-//     "type": "TAG",
+//     "type": "EventName",
 //     "attrs": [
 //       {
 //         "key": "key",
 //         "value": "foo"
 //       }
 //     ],
-//     "pkg_path": "gno.land/r/demo/event",
+//     "pkg_path": "gno.land/r/demo/emit",
 //     "func": "Emit"
 //   },
 //   {
-//     "type": "TAG",
+//     "type": "EventName",
 //     "attrs": [
 //       {
 //         "key": "key",
 //         "value": "bar"
 //       }
 //     ],
-//     "pkg_path": "gno.land/r/demo/event",
+//     "pkg_path": "gno.land/r/demo/emit",
 //     "func": "Emit"
 //   }
 // ]

examples/gno.land/r/demo/hello_world/gno.mod

@@ -0,0 +1 @@
+module gno.land/r/demo/hello_world

examples/gno.land/r/demo/hello_world/hello.gno

@@ -0,0 +1,17 @@
+// Package hello_world demonstrates the usage of the Render() function.
+// Render() can be called via the vm/qrender ABCI query off-chain to
+// retrieve realm state or any other custom data defined by the realm
+// developer. The vm/qrender query allows for additional data to be
+// passed in with the call, which can be utilized as the path argument
+// to the Render() function. This allows developers to create different
+// "renders" of their realms depending on the data which is passed in,
+// such as pagination, admin dashboards, and more.
+package hello_world
+
+func Render(path string) string {
+	if path == "" {
+		return "# Hello, 世界！"
+	}
+
+	return "# Hello, " + path + "!"
+}

examples/gno.land/r/demo/hello_world/hello_test.gno

@@ -0,0 +1,19 @@
+package hello_world
+
+import (
+	"testing"
+)
+
+func TestHello(t *testing.T) {
+	expected := "# Hello, 世界！"
+	got := Render("")
+	if got != expected {
+		t.Fatalf("Expected %s, got %s", expected, got)
+	}
+
+	got = Render("world")
+	expected = "# Hello, world!"
+	if got != expected {
+		t.Fatalf("Expected %s, got %s", expected, got)
+	}
+}