Added page for deep linking util

docs/utils/deep_linking.rst

@@ -0,0 +1,60 @@
+============
+Deep Linking
+============
+
+Telegram bots have a deep linking mechanism, that allows for passing
+additional parameters to the bot on startup. It could be a command that
+launches the bot — or an auth token to connect the user's Telegram
+account to their account on some external service.
+
+You can read detailed description in the source:
+https://core.telegram.org/bots#deep-linking
+
+We have added some utils to get deep links more handy.
+
+Examples
+========
+
+Basic link example
+------------------
+
+.. code-block:: python
+
+    from aiogram.utils.deep_linking import create_start_link
+
+    link = await create_start_link(bot, 'foo')
+
+    # result: 'https://t.me/MyBot?start=foo'
+
+Encoded link
+------------
+
+.. code-block:: python
+
+    from aiogram.utils.deep_linking import create_start_link
+
+    link = await create_start_link(bot, 'foo', encode=True)
+    # result: 'https://t.me/MyBot?start=Zm9v'
+
+Decode it back
+--------------
+
+.. code-block:: python
+
+    from aiogram.utils.deep_linking import decode_payload
+    from aiogram.filters import CommandStart, CommandObject
+    from aiogram.types import Message
+
+    @router.message(CommandStart(deep_link=True))
+    async def handler(message: Message, command: CommandObject):
+        args = command.args
+        payload = decode_payload(args)
+        await message.answer(f"Your payload: {payload}")
+
+
+References
+==========
+
+.. autofunction:: aiogram.utils.deep_linking.create_start_link
+
+.. autofunction:: aiogram.utils.deep_linking.decode_payload