[Docs] Red backup and restore instructions. (#6654)

docs/backup_red.rst

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+.. _backup-red:
+
+============================
+Backing Up and Restoring Red
+============================
+
+Red can be backed up and restored to any device as long as it is supported operating system. See page: :ref:`end-user-guarantees`.
+
+Backup steps are to be done in order and carefully to avoid any issues.
+
+#. Take note of the installed cogs with ``[p]cogs``; and cog repositories with ``[p]load downloader``, then ``[p]repo list`` (``[p]`` is your bot's prefix).
+#. Stop the bot, ideally with ``[p]shutdown``.
+#. Activate your venv, and run ``redbot-setup backup <instancename>``, replacing ``<instancename>`` with the name of your instance.
+#. Copy your backup file to the new machine/location.
+#. Extract the file to a location of your choice (remember the full path and make sure that the user you are going to install/run Red under can access this path).
+#. :ref:`Install Red <install-guides>` as normal on the new machine/location.
+#. Run ``redbot-setup`` in your venv to create a new instance, using the path you remembered above as your data path.
+#. Start your new instance.
+#. Re-add the cog repositories using the same names as before.
+#. Do ``[p]cog update``.
+#. Re-add any cogs that were not re-installed (you may have to uninstall them first as Downloader may think they are still installed).
+
+    .. note::
+
+        The config (data) from cogs has been saved, but not the code itself.
+
+    .. tip::
+
+        You can fix permissions (if needed) on your directory using:
+
+        .. code-block:: bash
+
+            sudo chown -R <user>:<user> ~/.local
+
+        Replace ``<user>`` with your actual username.