docs site: add PR previews (#865)

documentation/docusaurus.config.ts

@@ -20,7 +20,7 @@ const config: Config = {
   url: "https://block.github.io/",
   // Set the /<baseUrl>/ pathname under which your site is served
   // For GitHub pages deployment, it is often '/<projectName>/'
-  baseUrl: "/goose/",
+  baseUrl: process.env.TARGET_PATH || "/goose/",
 
   // GitHub pages deployment config.
   // If you aren't using GitHub pages, you don't need these.