Add wiki navigation sidebar

_Sidebar.md

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+### Rivencraft
+
+- [Home](Home)
+- [Installation](Installation)
+- [Getting Started](Getting-Started)
+- [Books and Linking](Books-and-Linking)
+- [Writing Ages](Writing-Ages)
+- [Server and Operator Guide](Server-and-Operator-Guide)
+- [Contributing to the Wiki](Contributing-to-the-Wiki)
+
+[Source](https://git.paddedcell.org/dnorberg/rivencraft) · [Issues](https://git.paddedcell.org/dnorberg/rivencraft/issues) · [Releases](https://git.paddedcell.org/dnorberg/rivencraft/releases)