chore: prepare for docs sidebar

lib/load/docs/sidebars.js

@@ -0,0 +1,34 @@
+const fs = require('fs-extra');
+const path = require('path');
+const {idx} = require('../utils');
+
+module.exports = function loadSidebars({siteDir, env}) {
+  let allSidebars = {};
+
+  // current sidebars
+  const sidebarsJSONFile = path.join(siteDir, 'sidebars.json');
+  if (fs.existsSync(sidebarsJSONFile)) {
+    allSidebars = require(sidebarsJSONFile);
+  }
+
+  // versioned sidebars
+  if (idx(env, ['versioning', 'enabled'])) {
+    const versions = idx(env, ['versioning', 'versions']);
+    versions &&
+      versions.forEach(version => {
+        const versionedSidebarsJSONFile = path.join(
+          siteDir,
+          'versioned_sidebars',
+          `version-${version}-sidebars.json`
+        );
+        if (fs.existsSync(versionedSidebarsJSONFile)) {
+          const sidebar = require(versionedSidebarsJSONFile);
+          Object.assign(allSidebars, sidebar);
+        } else {
+          const missingFile = path.relative(siteDir, versionedSidebarsJSONFile);
+          throw new Error(`Failed to load ${missingFile}. It does not exist.`);
+        }
+      });
+  }
+  return allSidebars;
+};