docs: user guide pages, published to the GitHub wiki

docs/ gains pages for people using the app rather than building it:
getting started, installation, the user guide, account and sync,
Android and iOS, the command line, privacy, an FAQ and troubleshooting.
docs/README.md is the wiki's home page.

scripts/publish-wiki.sh, ported from proton-drive-linux, checks every
link in docs/ and README.md, rewrites links between pages to wiki page
names and copies the images they show. The Wiki workflow runs the check
on pull requests and publishes on every push to main.

images/ holds library, title and episode screenshots from the desktop
and Android apps, and the logo.

docs/README.md

@@ -0,0 +1,41 @@
+# proton-stream documentation
+
+proton-stream turns the videos in your Proton Drive, and the share links people send you, into
+a library you can browse and stream. It runs on Linux, Windows, Android, Android TV and iOS.
+This is an unofficial client, not a Proton product.
+
+<img src="../images/desktop-home.png" alt="The library in the desktop app" width="820">
+
+## Start here
+
+- [Getting started](GETTING_STARTED.md): install, add your first share, press play.
+- [Installation](INSTALL.md): every platform, and how to update and uninstall.
+- [FAQ](FAQ.md): short answers to the questions people ask first.
+
+## Using it
+
+- [User guide](USER_GUIDE.md): the library, title pages, the player, downloads, history and
+  settings.
+- [Account and sync](ACCOUNT_AND_SYNC.md): signing in, folders of your own Drive, and keeping
+  your place on every device.
+- [Android and iOS](MOBILE.md): what is different on a phone, a tablet and Android TV.
+
+## Reference
+
+- [Command-line reference](CLI.md): the `pstr` tool and the `proton-stream` launcher.
+- [Privacy](PRIVACY.md): what goes where, and what stays on your device.
+
+## When something goes wrong
+
+- [Troubleshooting](TROUBLESHOOTING.md): links that will not add, sign-in problems, playback
+  problems, and where the logs are.
+
+## Contributing
+
+- [Development](DEVELOPMENT.md): the state of each layer, the build order and the traps.
+- [Architecture](ARCHITECTURE.md): the crates, the data flow when an episode plays, and where
+  secrets live.
+- [Testing](TESTING.md), [Metadata](METADATA.md), [Design tokens](DESIGN-TOKENS.md),
+  [Android development](ANDROID.md) and [iOS development](IOS.md).
+
+Bugs are tracked in [BUGS.md](BUGS.md) in the repository.