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/firefox.md

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+# Firefox
+
+`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).
+
+## 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
+
+`prefs` and the fixes are *default* values (`pref()`), not user values:
+`about:config` can still change them per profile, and nothing is written to
+`prefs.js`, so removing one leaves nothing behind. Firefox reads default
+prefs from `defaults/pref/*.js` in its system config dir, which in the
+Flatpak is `/app/etc/firefox`, the mount point of the
+`org.mozilla.firefox.systemconfig` extension. home-manager provides that
+extension as a link from
+`~/.local/share/flatpak/extension/org.mozilla.firefox.systemconfig/aarch64/stable`
+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
+`/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
+from it in `prefs.js`. A second launch that just hands a URL to the running
+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 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.