docs: add snippet for qt compile_ method generated inputs

docs/markdown/snippets/qt_module_generated_inputs.md

@@ -0,0 +1,13 @@
+## The qt modules now accept generated outputs as inputs for qt.compile_*
+
+This means you can uset `custom_target`, custom_target indecies
+(`custom_target[0]`, for example), or the output of `generator.process` as
+inputs to the various `qt.compile_*` methods.
+
+```meson
+qt = import('qt5')
+
+ct = custom_target(...)
+
+out = qt.compile_ui(sources : ct)
+```