Update and rename index.rst to index.md

Author: TimGoTheCreator

docs/index.md

@@ -0,0 +1,15 @@
+# NEXT Documentation
+
+Welcome to the documentation for **NEXT: Newtonian EXact Trajectories**.
+
+## Contents
+
+```{toctree}
+:maxdepth: 2
+:caption: Sections
+
+getting_started
+usage
+theory
+api
+citation