docs: hyperlink buttons demo (#3245)

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

## Description

Adds a `r/docs` that showcases how "buttons" can be added to realm
renders.

<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>

---------

Co-authored-by: Morgan <[email protected]>

examples/gno.land/r/docs/buttons/buttons.gno

@@ -0,0 +1,44 @@
+package buttons
+
+import (
+	"std"
+
+	"gno.land/p/demo/ufmt"
+	"gno.land/p/moul/txlink"
+)
+
+var (
+	motd       = "The Initial Message\n\n"
+	lastCaller std.Address
+)
+
+func UpdateMOTD(newmotd string) {
+	motd = newmotd
+	lastCaller = std.PrevRealm().Addr()
+}
+
+func Render(path string) string {
+	if path == "motd" {
+		out := "# Message of the Day:\n\n"
+		out += "---\n\n"
+		out += "# " + motd + "\n\n"
+		out += "---\n\n"
+		link := txlink.Call("UpdateMOTD", "newmotd", "Message!") // "/r/docs/buttons$help&func=UpdateMOTD&newmotd=Message!"
+		out += ufmt.Sprintf("Click **[here](%s)** to update the Message of The Day!\n\n", link)
+		out += "[Go back to home page](/r/docs/buttons)\n\n"
+		out += "Last updated by " + lastCaller.String()
+
+		return out
+	}
+
+	out := `# Buttons
+
+Users can create simple hyperlink buttons to view specific realm pages and
+do specific realm actions, such as calling a specific function with some arguments.
+
+The foundation for this functionality are markdown links; for example, you can
+click...
+` + "\n## [here](/r/docs/buttons:motd)\n" + `...to view this realm's message of the day.`
+
+	return out
+}

examples/gno.land/r/docs/buttons/buttons_test.gno

@@ -0,0 +1,14 @@
+package buttons
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRenderMotdLink(t *testing.T) {
+	res := Render("motd")
+	const wantLink = "/r/docs/buttons$help&func=UpdateMOTD&newmotd=Message!"
+	if !strings.Contains(res, wantLink) {
+		t.Fatalf("%s\ndoes not contain correct help page link: %s", res, wantLink)
+	}
+}

examples/gno.land/r/docs/buttons/gno.mod

@@ -0,0 +1 @@
+module gno.land/r/docs/buttons

examples/gno.land/r/docs/docs.gno

@@ -11,6 +11,7 @@ Explore various examples to learn more about Gno functionality and usage.
 - [Hello World](/r/docs/hello) - A simple introductory example.
 - [Adder](/r/docs/adder) - An interactive example to update a number with transactions.
 - [Source](/r/docs/source) - View realm source code.
+- [Buttons](/r/docs/buttons) - Add buttons to your realm's render.
 - [AVL Pager](/r/docs/avl_pager) - Paginate through AVL tree items.  
 - [Img Embed](/r/docs/img_embed) - Demonstrates how to embed an image.
 - ...