feat(v2): support syncing tab choices (#2366)

* feat(v2): Support syncing tab choices

* Move docs changes to website/docs

* Do not import entire React in TabGroupChoiceContext

* Store only one tab choice according to discussion in PR

* Remove leftover logging code during debugging

* Put storage value in separate const outside the hook-level

* Use an array to keep track of different tab groups

* Revert back to using `groupId`

* Update markdown-features.mdx

* Update markdown-features.mdx

Co-authored-by: Yangshun Tay <tay.yang.shun@gmail.com>

packages/docusaurus-theme-classic/src/theme/hooks/useTabGroupChoice.js

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {useState, useCallback, useEffect} from 'react';
+
+const TAB_CHOICE_PREFIX = 'docusaurus.tab.';
+
+const useTabGroupChoice = () => {
+  const [tabGroupChoices, setChoices] = useState({});
+  const setChoiceSyncWithLocalStorage = useCallback((groupId, newChoice) => {
+    try {
+      localStorage.setItem(`${TAB_CHOICE_PREFIX}${groupId}`, newChoice);
+    } catch (err) {
+      console.error(err);
+    }
+  }, []);
+
+  useEffect(() => {
+    try {
+      const localStorageChoices = {};
+      for (let i = 0; i < localStorage.length; i += 1) {
+        const storageKey = localStorage.key(i);
+        if (storageKey.startsWith(TAB_CHOICE_PREFIX)) {
+          const groupId = storageKey.substring(TAB_CHOICE_PREFIX.length);
+          localStorageChoices[groupId] = localStorage.getItem(storageKey);
+        }
+      }
+      setChoices(localStorageChoices);
+    } catch (err) {
+      console.error(err);
+    }
+  }, []);
+
+  return {
+    tabGroupChoices,
+    setTabGroupChoices: (groupId, newChoice) => {
+      setChoices(oldChoices => ({...oldChoices, [groupId]: newChoice}));
+      setChoiceSyncWithLocalStorage(groupId, newChoice);
+    },
+  };
+};
+
+export default useTabGroupChoice;