README: full user documentation again, docs/ only technical

The README is the complete user documentation again: features, install,
two sessions, usage, the full options table (and renamed options), every
feature with problem, usage, limitations, security and "remove when",
UI patches at user level (DevTools on the LAN, after a Steam update),
changes outside Nix, rollback and uninstall.

docs/ keeps only the technical side (how the patches work, writing
patches, signatures and the update procedure, the SteamVR debugger
mechanics, cleanup and installer internals), linked from each feature.
docs/options.md, two-sessions.md and desktop-integration.md are gone
(their content is in the README).

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 (docs/ui-patches.md,
+# stopped. Developer Mode forwards the port to 0.0.0.0:8088 (README,
 # "DevTools on the LAN"); our patches use 127.0.0.1 only.
 { config, lib, pkgs, ... }:
 let