Add sandbox project using Contentful CMS

I used the boilerplate/typescript project as a starting point. This project
fetches and renders books that I'm defining in a Contentful CMS that I created.

website/sandbox/contentful/README.md

@@ -0,0 +1,18 @@
+# Contentful
+
+I have not used a CMS in a few years. I learned about Contentful from a
+Gatsby.js tutorial, and I wanted to learn more; I created a Contentful account,
+and I'm experimenting with the data here.
+
+## Developing
+
+```shell
+$ nix-shell
+$ yarn run dev
+```
+
+## Building
+
+```shell
+$ nix-build
+```