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.

docs/steamvr-debugger.md

@@ -1,18 +1,37 @@
-# SteamVR debugger: how it works
+# SteamVR debugger
 
-`steamvrDebugger.enable` (module `steamvr-debugger`). What it is for and
-what you need to do: README,
-[SteamVR debugger](../README.md#steamvr-debugger-steamvrdebuggerenable).
+`steamvrDebugger.enable`, module `steamvr-debugger`. Automatic; nothing to
+set. Options: [README, Options](../README.md#options).
 
-Enabled automatically when any patch in `steamFrame.uiPatches.patches` has
-an `endpoint` on port 8087 (all [dashboard](dashboard.md) patches and the VR
-keyboard's strip below/above the keyboard).
+## Problem
 
-SteamVR opens its DevTools port 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. It is set only while
-SteamVR runs:
+The [dashboard patches](ui-patches.md#steamvr-dashboard-patches) (and the
+[VR keyboard](keyboard.md#swipe-and-suggestions)'s strip below/above the
+keyboard) need SteamVR's DevTools port, which SteamVR opens only with its
+setting `VRWebHelper/DebuggerEnabled`. That setting lives in
+`~/.config/openvr/config/steamvr.vrsettings`, which SteamVR rewrites from
+memory, so it can't be a Nix link and can't be edited while SteamVR runs.
+
+## What you get
+
+The port, enabled automatically when any patch in
+`steamFrame.uiPatches.patches` has an `endpoint` on port 8087. The setting
+is **set only while SteamVR runs**: set before every SteamVR start, put back
+to its previous value when SteamVR stops, also after a rollback or uninstall
+(without Nix). A value you set to `true` yourself is never touched.
+
+**The first time, restart SteamVR once** (e.g. reboot); until then the
+dashboard patches wait. Turned off, the setting is restored at the switch
+(or when SteamVR stops, if it runs).
+
+## 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)).
+
+## How it works
+
+Port: `VRWebHelper/DebuggerPort`, default 8087.
 
 - before every SteamVR start, the `steamvr-webhelper-debugger` oneshot
   (a drop-in on `steamvr.service`, running `install.sh steamvr-debugger-arm`)
@@ -25,12 +44,12 @@ SteamVR runs:
 - 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, and `.armed` (the only trace after a
+The runtime files don't need Nix, which is why rollback and uninstall are
+covered; they are gone at reboot, and `.armed` (the only trace after a
 power loss) is resolved by the next SteamVR start or
-`steam-frame-nix-cleanup`. A key set to `true` by the user is never touched.
-Disabled, there is no unit; the switch (cleanup) restores the key if SteamVR
-is stopped, otherwise the runtime drop-in does when it stops.
+`steam-frame-nix-cleanup`. Disabled, there is no unit; the switch (cleanup)
+restores the key if SteamVR is stopped, otherwise the runtime drop-in does
+when it stops.
 
 Until SteamVR has been restarted once with the drop-in, the port is closed
 and `steam-ui-patches` keeps polling it.