Restrucuture documentation to smaller parts

docs/users/download_segments.md

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+---
+parent: Using BRouter
+---
+
+# Download Routing Segments
+
+Routing data files are organised as 5\*5 degree files, with the filename
+containing the south-west corner of the square, which means:
+
+- You want to route near West48/North37 -> you need `W50_N35.rd5`
+- You want to route near East7/North47 -> you need `E5_N45.rd5`
+
+These data files, called _segments_ across BRouter, are generated from
+[OpenStreetMap](https://www.openstreetmap.org/) data and stored in a custom
+binary format (rd5) for improved efficiency of BRouter routing.
+
+## Download them from brouter.de
+
+Segments files from the whole planet are generated weekly at
+[https://brouter.de/brouter/segments4/](http://brouter.de/brouter/segments4/).
+
+You can download one or more segments files, covering the area of the planet
+your want to route, into the `segments4` directory.