docs: Prefix code documentation files

with the subsystem they belong to. This makes it easier to see which
files belong where.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3485>

doc/clutter-frame-scheduling.md

@@ -1,4 +1,4 @@
-# Frame scheduling
+# Clutter Frame scheduling
 
 `ClutterFrameClock` state diagram.
 

doc/code-overview.md

@@ -10,7 +10,7 @@ Hardware acceleration pipeline abstraction layer. Handles things like allocating
 
 Compositing toolkit, containing an actor and render node based scene graph, and has features such as input event routing, transformation and animation. Handles compositing, both Wayland surfaces, X11 windows, and is the basis of the UI toolkit implemented by [GNOME Shell](https://gitlab.gnome.org/GNOME/gnome-shell). Originally a fork of [the Clutter project](https://gitlab.gnome.org/GNOME/clutter).
 
-* [Frame Scheduling](frame-scheduling.md)
+* [Frame Scheduling](clutter-frame-scheduling.md)
 
 ## Mtk
 
@@ -21,5 +21,5 @@ The Meta Toolkit containing utilities shared by other parts of mutter.
 The display server and window manager library. Contains a X11 window manager and compositing manager implementation, as well as a Wayland display server implementation.
 
 * [Compositor stage and hardware relationships](mutter-relationships.md)
-* [Window constraints](how-constraints-works.txt)
+* [Window constraints](mutter-constraints.txt)
-* [How to get focus right](how-to-get-focus-right.txt)
+* [How to get focus right](mutter-focus.txt)