[docs] Brush up the documentation

* Try to organize after expected user need
* Update mkdocs stuff and fix a broken link
* Move web building info to building.md
* Add info about Roku Soundbridge
* Add note about Google Nest not working

docs/audio-outputs/streaming.md

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+# Streaming
+
+The streaming option is useful when you want to listen to audio played by
+OwnTone in a browser or a media player of your choice [^1],[^2].
+
+You can control playback via the web interface or any of the supported control
+clients.
+
+## Listening to Audio in a Media Player
+
+To listen to audio being played by OwnTone in a media player, follow these
+steps:
+
+1. Start playing audio in OwnTone.
+2. In the web interface, activate the stream in the output menu by clicking
+   on the icon :material-broadcast: next to HTTP Stream.
+   After a few seconds, the audio should play in the background.
+3. Copy the URL behind the :material-open-in-new: icon next to HTTP Stream.
+4. Open the copied URL with the media player, e.g., VLC.
+   The URL is usually
+   [http://owntone.local:3689/stream.mp3](http://owntone.local:3689/stream.mp3)
+   or http://SERVER_ADDRESS:3689/stream.mp3
+
+## Notes
+
+[^1]: On iOS devices, the streaming option is the only way of listening to your
+      audio, since Apple does not allow AirPlay receiver apps, and because
+      Home Sharing cannot be supported by OwnTone.
+
+[^2]: For the streaming option to work, MP3 encoding must be supported by
+      `libavcodec`. If it is not, a message will appear in the log file.
+      For example, on Debian or Ubuntu, MP3 encoding support is provided by the
+      package `libavcodec-extra`.