misc: enable jsx-key eslint rule (#6584)

.eslintrc.js

@@ -149,6 +149,7 @@ module.exports = {
       },
     ],
     'react/jsx-filename-extension': OFF,
+    'react/jsx-key': [ERROR, {checkFragmentShorthand: true}],
     'react/jsx-props-no-spreading': OFF,
     'react/no-array-index-key': OFF, // We build a static site, and nearly all components don't change.
     'react/no-unstable-nested-components': [WARNING, {allowAsProps: true}],

packages/docusaurus-theme-classic/src/theme/Tabs/__tests__/index.test.tsx

@@ -120,7 +120,9 @@ describe('Tabs', () => {
               values={tabs.map((t, idx) => ({label: t, value: idx}))}
               defaultValue={0}>
               {tabs.map((t, idx) => (
-                <TabItem value={idx}>{t}</TabItem>
+                <TabItem key={idx} value={idx}>
+                  {t}
+                </TabItem>
               ))}
             </Tabs>
           </TabGroupChoiceProvider>