add overview page for #2

src/SUMMARY.md

@@ -1,6 +1,7 @@
 # Summary
 
 - [Introduction](./intro.md)
+- [Overview](./overview.md)
 - [Example](./example.md)
 - [UTA Variables](./variables.md)
 - [Data Sources](./data.md)

src/overview.md

@@ -0,0 +1,17 @@
+# Overview
+
+This chapter provides an overview of the three main pages of the [Urban Analyst
+platform](https://urbananalyst.city):
+
+- [Maps](https://urbananalyst.city/maps) showing interactive maps for each
+  city;
+- [Stats](https://urbananalyst.city/stats) comparing aggregate statistics for
+  all cities; and
+- [Transformations](https://urbananalyst.city/transform) for transforming any
+  chosen city to become more like other cities.
+
+## Maps
+
+## Stats
+
+## Transformations