docs: oas (#197)

Adds OpenAPI specification of Storefront and Admin APIs.
Updates docs project.

docs-util/fixture-gen/utils/write-fixture.js

@@ -0,0 +1,30 @@
+const fs = require("fs").promises;
+const path = require("path");
+
+class FixtureWriter {
+  constructor() {
+    const resolvedPath = path.resolve("./docs/api/fixtures.json");
+    const existing = require(resolvedPath);
+
+    this.toWrite_ = existing.resources;
+    this.resolvedPath_ = resolvedPath;
+  }
+
+  addFixture(key, data) {
+    this.toWrite_ = {
+      ...this.toWrite_,
+      [key]: data,
+    };
+  }
+
+  async execute() {
+    const toSet = {
+      resources: this.toWrite_,
+    };
+    const s = JSON.stringify(toSet, null, 2);
+    return await fs.writeFile(this.resolvedPath_, s);
+  }
+}
+
+const instance = new FixtureWriter();
+export default instance;