Switch to mdbook

Signed-off-by: Jonas Zohren <git-pbkyr@jzohren.de>

docs/SUMMARY.md

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+# Summary
+
+[Home](index.md)
+
+- [Installation](installation.md)
+    - [Prerequisites](installation/prerequisites.md)
+    - [From Binaries](installation/manual.md)
+    - [From Packages](installation/packages.md)
+    - [With Docker](installation/docker.md)
+- [Configuration](configuration.md)
+    - [Config options](configuration/configuration.md)
+    - [Appservices](configuration/appservices.md)
+- [Contribute](contribute.md)
+    - [Basics](development/basics.md)
+    - [Cross compilation](development/cross-compilation.md)
+    - [Tests & CI](development/tests-ci.md)