docs: add dzgui7 onboarding page

docs/source/dzgui7.rst

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+Installation (DZGUI 7 beta)
+==================
+
+In order to make the first-time setup experience easier for end-users, DZGUI 7 no longer requires
+the tool to be installed from source code into a working Python environment. All necessary dependencies and the
+runtime environment are built-in.
+
+Turnkey installer
+------------------------
+
+- Visit the project's `Releases page <https://github.com/aclist/dztui/releases>`_.
+- Extract the DZGUI tarball from the top of the **Assets** list.
+- From a terminal, run the command ``./dzgui`` or double click on the extracted file in
+  a file explorer.
+- Once the setup wizard opens, follow the steps at the bottom of this page.
+
+UI dependencies (Python and PyGObject)
+---------------------------------------
+On most major distributions, GObject introspection libraries should be available by default.
+
+If you receive an error about a missing ``libgirepository-2.0`` dependency, install it via your system's package manager.
+
+On Debian-based distributions (Ubuntu, etc.), this package is not explicitly installed on the system.
+You can install it via the following set of commands:
+
+.. code:: console
+
+   sudo apt update
+   sudo apt install libgirepository-2.0
+
+Next steps
+---------------
+Proceed to the :doc:`setup` section for setup instructions.