[docs] Generate OwnTone documentation with MkDocs

- Copy existing README*.md and INSTALL.md files to docs folder
- Set up MkDocs to build documentation (mkdocs.yml)
- Add new index.md with a quick overview of OwnTone

docs/index.md

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+# OwnTone
+
+**OwnTone** is an open source (audio) media server for GNU/Linux, FreeBSD
+and MacOS.
+
+It allows sharing and streaming your media library to iTunes (DAAP[^1]),
+Roku (RSP), AirPlay devices (multiroom), Chromecast and also supports local playback.
+
+You can control OwnTone via its web interface, Apple Remote (and compatible
+DAAP/DACP clients), MPD clients or via its JSON API.
+
+Besides serving your local music, podcast and audiobook media files,
+OwnTone supports internet radios and Spotify (requires Spotify premium account).
+
+OwnTone was previously called forked-daapd, which again was a rewrite of
+mt-daapd (Firefly Media Server). It is written in C with a web interface written in Vue.js.
+
+[^1]:
+    DAAP stands for Digital Audio Access Protocol which is the protocol used by
+    iTunes and friends to share/stream media libraries over the network.
+
+## Features
+
+- Stream to :material-cast-variant: AirPlay (synchronized multiroom) and :material-cast:
+  Chromecast devices
+- :material-music-box-multiple-outline: Share local library with iTunes and Roku
+- :material-volume-high: Local audio playback with ALSA or PulseAudio
+- Supports multiple different clients:
+
+    - :material-cellphone: Remote apps like Apple Remote (iOS) or Retune (Android)
+    - :material-web: Integrated mobile friendly web interface
+    - :material-console: MPD clients
+
+- Supports :material-music: music and :material-book-open-variant:
+  audiobook files, :material-microphone: podcast files and :material-rss: RSS
+  and :material-radio: internet radio
+- :material-file-music: Supports audio files in most formats
+- :material-spotify: Supports playing your Spotify library (requires
+  Spotify premium account)
+- :material-raspberry-pi: Runs on low power devices like the Raspberry Pi
+
+---
+
+![Now playing](images/screenshot-now-playing.png){: class="zoom" }
+![Music browse](images/screenshot-music-browse.png){: class="zoom" }
+![Music album](images/screenshot-music-album.png){: class="zoom" }
+
+_(You can find more screenshots from OwnTone's web interface [here](web-interface.md))_
+{: class="text-center" }
+
+---
+
+## Looking for help?
+
+Before you continue, make sure you know what version of OwnTone you have,
+and what features it was built with (e.g. Spotify support).
+
+How to find out? Go to the [web interface](http://owntone.local:3689) and
+check. No web interface? Then check the top of OwnTone's log file (usually
+/var/log/owntone.log).
+
+Note that you are viewing a snapshot of the instructions that may or may not
+match the version of OwnTone that you are using.
+
+If you are looking for help on building OwnTone (not using it), then
+please see the documentation on [Building from Source](installation.md).
+
+## References
+
+The source for this version of OwnTone can be found here:
+
+[https://github.com/owntone/owntone-server.git](https://github.com/owntone/owntone-server.git)