Fold the Steam Frame guide into the README, with a quick start

docs/steam-frame.md is gone. The README now opens with a quick start that
goes from the disc to playing (emulation setup, disc extraction, the
container build, installing with Frame Control, updating), and carries what
the guide held that a player or contributor needs: recommended settings, the
Frame controller map, known issues, troubleshooting (log lines, the pacing
line, crash backtraces), building on the Frame or with Docker (including
Unraid's qemu registration), how the native build works and why the Android
flavour cannot show a picture. Links to the old guide point at the README.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

Launcher/build-dawn-linux.sh

@@ -3,7 +3,7 @@
 # Vulkan hooks through which the OpenXR runtime creates Dawn's own instance and device (the
 # same-device OpenXR backend, runtime/src/vr/openxr_vulkan_win32.cpp) and fragment density maps for
 # foveated rendering. The stock prebuilt package has neither. This is the Linux counterpart of
-# android/Build-QuestDawn.ps1, for the Steam Frame's native SteamOS build (docs/steam-frame.md).
+# android/Build-QuestDawn.ps1, for the Steam Frame's native SteamOS build (README.md, Quick start).
 #
 #   Launcher/build-dawn-linux.sh [--work-dir DIR] [--cc PATH --cxx PATH] [--cmake PATH]
 #                                [--ninja PATH] [--python PATH] [--jobs N] [--force]