docs: split feature details into docs/

One page per feature group (sessions, keyboard, launcher menu, dashboard, SteamVR debugger, Firefox, Jellyfin, UI patches, changes outside Nix) and the full options table in docs/options.md.

docs/steamvr-debugger.md

@@ -0,0 +1,36 @@
+# SteamVR debugger
+
+`steamvrDebugger.enable`, automatic: on when any patch in
+`steamFrame.uiPatches.patches` uses port 8087 (all [dashboard](dashboard.md)
+patches and the VR keyboard's strip below/above the keyboard).
+
+**Problem:** dashboard patches need SteamVR's DevTools port, opened only
+with `VRWebHelper/DebuggerEnabled` (port `VRWebHelper/DebuggerPort`, default
+8087). SteamVR rewrites `~/.config/openvr/config/steamvr.vrsettings` from
+memory, so the key can't be a link and can't be edited while SteamVR runs.
+
+**What it does:** sets the key only while SteamVR runs:
+
+- before every SteamVR start, the `steamvr-webhelper-debugger` oneshot
+  (a drop-in on `steamvr.service`) stores the key's value in
+  `~/.local/state/steam-frame-nix/steamvr-debugger.armed`, sets the key
+  (with `jq`) and writes a runtime drop-in,
+  `/run/user/1000/systemd/user/steamvr.service.d/50-steam-frame-nix-debugger.conf`,
+  whose `ExecStopPost=` runs a restore script next to it
+  (`/run/user/1000/steam-frame-nix/`, `/usr/bin` tools only);
+- when SteamVR stops, the key goes back to its previous value (removed if
+  it wasn't there) and `.armed` is removed.
+
+The runtime files don't need Nix, so this also works after a rollback or
+uninstall; they are gone at reboot. A key you set to `true` yourself is
+never touched. Disabled, there is no unit; the switch restores the key if
+SteamVR is stopped, otherwise the runtime drop-in does when it stops.
+
+**The first time, restart SteamVR once** (e.g. reboot); until then
+`steam-ui-patches` keeps polling.
+
+**Security:** the port listens on `127.0.0.1` only; keep Developer Mode off
+(see [DevTools on the LAN](ui-patches.md#devtools-on-the-lan)).
+
+What it writes and when it is removed:
+[changes outside Nix](changes-outside-nix.md).