docs: serve backend metadata as JSON on the website

Add a Hugo page at /backends/index.json that exports all the
backend YAML data from docs/data/backends/ as a single JSON file
for external consumption.

docs/content/backends.md

@@ -0,0 +1,6 @@
+---
+title: "Backends"
+type: "backends"
+outputs:
+  - JSON
+---

docs/layouts/backends/single.json

@@ -0,0 +1 @@
+{{- hugo.Data.backends | jsonify (dict "indent" "  ") -}}