example: Add example to start bot with async context manager

Add basic example to start the bot with an async context manager.
The example contains a `/hello` slash command which responds with a
"Hellp @author".

examples/basic_async_bot.py

@@ -0,0 +1,32 @@
+import asyncio
+
+import discord
+from discord.ext import commands
+
+class Bot(commands.AutoShardedBot):
+    def __init__(self):
+        super().__init__(
+            command_prefix="!",
+            intents=discord.Intents.default(),
+        )
+
+    async def on_ready(self):
+        print("Ready called!")
+
+def main():
+    asyncio.run(start_bot_session())
+
+async def start_bot_session():
+    async with Bot() as bot:
+        # NOTE:
+        #   Better add those slash commands to a cog (check examples/app_commands/slash_cog).
+        #   But for the purpose of simplicity, we just define the command here.
+        @bot.slash_command(guild_ids=[...])  # Create a slash command.
+        async def hello(ctx: discord.ApplicationContext):
+            """Say hello to the bot"""  # The command description can be supplied as the docstring
+            await ctx.respond(f"Hello {ctx.author.mention}!")
+
+        await bot.start("TOKEN")
+
+if __name__ == "__main__":
+    main()