docs: Move kms abstractions documentation out of references

Eventually we want to have all the high-level code documentation in the
component API reference documentation. However, gi-docgen is currently
missing support for mermaid so we just keep the files in `doc/` and link
to them from `code-overview.md`.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3505>

doc/code-overview.md

@@ -22,5 +22,6 @@ 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)
+* [KMS abstraction](mutter-kms-abstractions.md)
 * [Window constraints](mutter-constraints.txt)
 * [How to get focus right](mutter-focus.txt)

doc/mutter-kms-abstractions.md

@@ -1,4 +1,4 @@
-Title: KMS abstraction
+# KMS abstraction
 
 The KMS abstraction consists of various building blocks for helping out with
 interacting with the various drm API's, enabling users to use a

doc/reference/meta/meta.toml.in

@@ -39,7 +39,6 @@ base_url = "https://gitlab.gnome.org/GNOME/mutter/-/blob/@vcs_tag@/"
 [extra]
 # The same order will be used when generating the index
 content_files = [
-  "kms-abstractions.md",
 ]
 content_images = [
   "logo.svg",