docs: Add Setup page

docs/setup.rst

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+=====
+Setup
+=====
+
+Use gtk-blueprint-tool in your project
+--------------------------------------
+
+gtk-blueprint-tool works as a meson subproject.
+
+#. Save the following file as ``subprojects/gtk-blueprint-tool.wrap``:
+
+   .. code-block:: cfg
+
+      [wrap-git]
+      directory = gtk-blueprint-tool
+      url = https://gitlab.gnome.org/jwestman/gtk-blueprint-tool.git
+      revision = main
+      depth = 1
+
+      [provide]
+      program_names = gtk-blueprint-tool
+
+#. Add this to your ``.gitignore``:
+
+   .. code-block::
+
+      /subprojects/gtk-blueprint-tool
+
+#. Add this to the ``meson.build`` file where you build your GResources:
+
+   .. code-block:: meson.build
+
+      blueprints = custom_target('blueprints',
+        input: files(
+          # LIST YOUR BLUEPRINT FILES HERE
+        ),
+        output: '.',
+        command: [find_program('gtk-blueprint-tool'), 'batch-compile', '@OUTPUT@', '@INPUT@'],
+      )
+
+#. In the same ``meson.build`` file, add this argument to your ``gnome.compile_resources`` command:
+
+   .. code-block:: meson.build
+
+      dependencies: blueprints,
+
+#. Convert your .ui XML files to blueprint format. In the future, an automatic
+   porting tool is planned.
+
+
+.. warning::
+   The blueprint compiler flattens the directory structure of the resulting XML
+   files. You may need to update your ``.gresource.xml`` file to match.