fix(v2): alpha 62 doc fixes (#3381)

* deprecated nextVersionLabel option

* useActivePlugin failfast option

* remove deprecated option nextVersionLabel

* routeBasePath: '' should be forbidden

* routeBasePath: '' should be forbidden

* Docs: do not show version badge if there is only 1 version: https://github.com/facebook/docusaurus/issues/3362

* allow sidebars file to not exist: fallback to empty sidebars
https://githu.com/facebook/docusaurus/issues/3366

packages/docusaurus-plugin-content-docs/src/__tests__/sidebars.test.ts

@@ -115,9 +115,13 @@ describe('loadSidebars', () => {
   });
 
   test('unexisting path', () => {
+    /*
     expect(() => loadSidebars('badpath')).toThrowErrorMatchingInlineSnapshot(
       `"No sidebar file exist at path: badpath"`,
     );
+     */
+    // See https://github.com/facebook/docusaurus/issues/3366
+    expect(loadSidebars('badpath')).toEqual({});
   });
 
   test('undefined path', () => {