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).

docs/firefox.md

@@ -1,31 +1,9 @@
-# Firefox
+# Firefox: how it works
 
-`firefox.enable`: a launcher for the Flathub Firefox Flatpak
-(`org.mozilla.firefox`, stable). It shadows the Flatpak's own entry (same
-ID), so default-browser associations keep working. Options:
-[options.md#firefox](options.md#firefox).
+`firefox.*` (module `firefox`). What it does and how to configure it:
+README, [Firefox](../README.md#firefox-firefox).
 
-## Options
-
-- **`vrFullscreenFix`** (on): gamescope never shows fullscreen windows, so
-  Firefox looks frozen. `full-screen-api.ignore-widgets` makes fullscreen
-  fill just the window. Not applied in the desktop profile. **Remove when**
-  gamescope shows fullscreen X11 windows in VR.
-- **`disableAv1`** (off): `media.av1.enabled = false`. The Frame's decoder
-  driver (`iris`) has no AV1, only H.264, HEVC and VP9, so YouTube and co.
-  send VP9/H.264, decoded in hardware, instead of software AV1. **Remove
-  when** a SteamOS kernel adds AV1 to `iris`.
-- **`prefs`:** further `about:config` values for every profile; they can
-  also override the fixes above.
-- **`desktopProfile`** (`"desktop"`): the sessions can't see each other's
-  Firefox, so a second instance stops at the locked profile; in the nested
-  desktop the launcher uses this separate profile (a normal Firefox profile
-  with its own browser data, created on first use). `null`: the default
-  profile in both sessions.
-
-Changes take effect at the next start of Firefox.
-
-## How it works
+## Default prefs
 
 `prefs` and the fixes are *default* values (`pref()`), not user values:
 `about:config` can still change them per profile, and nothing is written to
@@ -38,8 +16,12 @@ extension as a link from
 to a store directory; Flatpak mounts it itself, so the sandbox doesn't get
 `/nix`. (A systemconfig extension of your own would conflict with it.)
 
-The desktop profile undoes the fullscreen fix only while its Firefox runs:
-the launcher links the profile's `user.js` to
+## Desktop profile
+
+The launcher (a desktop entry with the Flatpak's ID, shadowing its entry)
+picks `desktopProfile` when started in the nested desktop. The desktop
+profile undoes the fullscreen fix only while its Firefox runs: the launcher
+links the profile's `user.js` to
 `/app/etc/firefox/steam-frame-nix-desktop-user.js` (a sandbox path) right
 before starting Firefox, waits for it, and once it has exited and the
 profile is no longer in use removes the link and the value Firefox stored
@@ -48,6 +30,8 @@ Firefox leaves both in place. A `user.js` of your own is never touched (the
 fix then stays on in that profile). After a crash, the next launch or
 `steam-frame-nix-cleanup` (on switch) removes them.
 
+## Older versions
+
 Older versions linked or copied a `user.js` into every profile; those and
 the values they left in `prefs.js` are removed by `steam-frame-nix-cleanup`
 on switch, for each profile not in use at that moment.