adopt myst

run rst2myst, with minimal manual formatting fixes

docs/topic/index.md

@@ -0,0 +1,19 @@
+# Topic Guides
+
+Topic guides provide in-depth explanations of specific topics.
+
+```{toctree}
+:caption: Topic guides
+:titlesonly: true
+
+whentouse
+requirements
+security
+customizing-installer
+installer-actions
+tljh-config
+authenticator-configuration
+escape-hatch
+idle-culler
+jupyterhub-configurator
+```