[docs] Split docs into multiple files and publish with gh-actions

docs/advanced/multiple-instances.md

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+# Running Multiple Instances
+
+To run multiple instances of owntone on a server, you should copy
+`/etc/owntone.conf` to `/etc/owntone-zone.conf` (for each `zone`) and
+modify the following to be unique across all instances:
+
+* the three port settings (`general` -> `websocket_port`,
+  `library` -> `port`, and `mpd` -> `port`)
+
+* the database paths (`general` -> `db_path`, `db_backup_path`, and `db_cache_path`)
+
+* the service name (`library` -> `name`).
+
+* you probably also want to disable local output (set `audio` -> `type =
+  "disabled"`).
+
+Then run `owntone -c /etc/owntone-zone.conf` to run owntone with the new
+zone configuration.
+
+Owntone has a `systemd` template which lets you run this automatically
+on systems that use systemd.  You can start or enable the service for
+a `zone` by `sudo systemctl start owntone@zone` and check that it is
+running with `sudo systemctl status owntone@zone`.  Use `sudo
+systemctl enable ownton@zone` to get the service to start on reboot.
+