doc: replace Cartridge CLI with tt CLI

Add deprecation note
Provide migration instructions

Author: p7nov

doc/index.rst

@@ -1,13 +1,20 @@
 Cartridge Command Line Interface
 ================================
 
+.. important::
+
+    Cartridge CLI is deprecated in favor of `tt CLI <https://www.tarantool.io/en/doc/latest/reference/tooling/tt_cli/>`_.
+    Find the instructions on switching from Cartridge CLI to ``tt`` in
+    :doc:` Migration from Cartridge CLI to tt <migration-to-tt>``.
+
 Control your Tarantool application instances via the command line.
 
 ..  toctree::
     :maxdepth: 2
     :includehidden:
 
     lifecycle
+    migration-to-tt
     installation
     commands
     global-flags

doc/migration-to-tt.rst

@@ -0,0 +1,51 @@
+Migration from Cartridge CLI to tt
+==================================
+
+To switch from Cartridge CLI to ``tt``
+
+To start managing a Cartridge application with ``tt`` instead of Cartridge CLI,
+run ``tt init`` in the application directory:
+
+.. code-block::bash
+
+    $ tt init
+    • Found existing config '.cartridge.yml'
+    • Environment config is written to 'tt.yaml'
+
+This creates a ``tt`` environment based on the existing Cartridge configuration.
+Now you're ready to manage the application with ``tt``:
+
+.. code-block::bash
+
+    $ tt start
+    • Starting an instance [app:s1-master]...
+    • Starting an instance [app:s1-replica]...
+    • Starting an instance [app:s2-master]...
+    • Starting an instance [app:s2-replica]...
+    • Starting an instance [app:stateboard]...
+    • Starting an instance [app:router]...
+    $ tt status
+    • app:router: RUNNING. PID: 13188.
+    • app:s1-master: RUNNING. PID: 13177.
+    • app:s1-replica: RUNNING. PID: 13178.
+    • app:s2-master: RUNNING. PID: 13179.
+    • app:s2-replica: RUNNING. PID: 13180.
+    • app:stateboard: RUNNING. PID: 13182.
+
+Commands difference
+-------------------
+
+Most Cartridge CLI commands look the same in ``tt``: ``cartridge start`` and
+``tt start``, ``cartridge create`` and ``tt create``, and so on. To migrate such
+calls, it is usually enough to replace the utility name. There can be slight differences
+in command flags and format. For details on ``tt`` commands, see the
+:ref:`tt commands reference <tt-commands>`.
+
+The following commands are different in ``tt``:
+
+*   Cartridge CLI commands ``admin``, ``bench``, ``failover``, ``repair``, ``replicasets``
+    are implemented as subcommands of ``tt cartridge``. Example, ``tt cartridge repair``.
+*   ``cartridge enter`` and ``cartridge connect`` are covered by ``tt connect``.
+*   The analog of ``cartridge gen completion`` is ``tt completion``
+*   ``cartridge log`` and ``cartridge pack docker`` functionality is not supported in ``tt``.
+