docs(examples): add echo bot demonstrating the handler framework

A minimal, runnable bot: Recover+Timeout middleware, /start via OnCommand, and
an echo handler guarded by HasText + Not(HasPrefix("/")) that replies with the
message text. Exercises the full incoming pipe end to end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ernado

examples/echo/main.go

@@ -0,0 +1,61 @@
+// Command echo is a minimal bot built on github.com/gotd/botapi: it greets on
+// /start and echoes any other text message back as a reply.
+//
+// Run it with an MTProto app identity (https://my.telegram.org) and a BotFather
+// token:
+//
+//	APP_ID=12345 APP_HASH=abcdef BOT_TOKEN=123:abc go run ./examples/echo
+package main
+
+import (
+	"context"
+	"os"
+	"os/signal"
+	"strconv"
+	"time"
+
+	"go.uber.org/zap"
+
+	"github.com/gotd/botapi"
+)
+
+func main() {
+	log, _ := zap.NewDevelopment()
+	defer func() { _ = log.Sync() }()
+
+	appID, err := strconv.Atoi(os.Getenv("APP_ID"))
+	if err != nil {
+		log.Fatal("APP_ID must be a number (see https://my.telegram.org)", zap.Error(err))
+	}
+
+	bot, err := botapi.New(os.Getenv("BOT_TOKEN"), botapi.Options{
+		AppID:   appID,
+		AppHash: os.Getenv("APP_HASH"),
+		Logger:  log,
+	})
+	if err != nil {
+		log.Fatal("Create bot", zap.Error(err))
+	}
+
+	// Middleware applies to every handler.
+	bot.Use(botapi.Recover(), botapi.Timeout(30*time.Second))
+
+	bot.OnCommand("start", func(c *botapi.Context) error {
+		_, err := c.Reply("Hi! Send me any text and I'll echo it back.")
+		return err
+	})
+
+	// Any text message that is not a command.
+	bot.OnMessage(func(c *botapi.Context) error {
+		_, err := c.Reply(c.Message().Text)
+		return err
+	}, botapi.HasText(), botapi.Not(botapi.HasPrefix("/")))
+
+	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
+	defer cancel()
+
+	log.Info("Starting echo bot")
+	if err := bot.Run(ctx); err != nil {
+		log.Fatal("Run", zap.Error(err))
+	}
+}