docs: README as overview + options, one docs page per feature

README keeps intro, feature list linking docs/, install, two sessions,
usage, the complete options table, changes outside Nix, rollback,
uninstall. Each feature's details (problem, what you get, configuration,
limitations, how it works) move to its own page in docs/; docs/dashboard.md
is split per feature and docs/changes-outside-nix.md becomes
docs/cleanup.md.

modules/steamvr-debugger.nix

@@ -20,7 +20,7 @@
 #   trace after a power loss) is resolved by the next start or
 #   steam-frame-nix-cleanup.
 # Off: no unit and no drop-in; cleanup restores the key once SteamVR is
-# stopped. Developer Mode forwards the port to 0.0.0.0:8088 (README,
+# stopped. Developer Mode forwards the port to 0.0.0.0:8088 (docs/ui-patches.md,
 # "DevTools on the LAN"); our patches use 127.0.0.1 only.
 { config, lib, pkgs, ... }:
 let