docs: Add a page for qt6

Which has been missing since the module was added in 0.57.0.
This also uses the include syntax

docs/markdown/Qt6-module.md

@@ -0,0 +1,8 @@
+# Qt6 module
+
+*New in Meson 0.57.0*
+
+The Qt5 module provides tools to automatically deal with the various
+tools and steps required for Qt.
+
+{{ _include_qt_base.md }}

docs/sitemap.txt

@@ -50,6 +50,7 @@ index.md
 			Python-module.md
 			Qt4-module.md
 			Qt5-module.md
+			Qt6-module.md
 			RPM-module.md
 			Rust-module.md
 			Simd-module.md