Add import statements and GitHub link in API docs (#2714)

docs/helpers.js

@@ -0,0 +1,10 @@
+const { version } = require('../package.json');
+
+module.exports = {
+  'github-link': (contractPath) => {
+    if (typeof contractPath !== 'string') {
+      throw new Error('Missing argument');
+    }
+    return `https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v${version}/contracts/${contractPath}`;
+  },
+};