fix(v2): sidebar_label should be used to compute next/previous button labels (#4970)

* sidebar_label should be used to compute next/previous button texts, as documented.

* improve docs frontmatter doc

* use a little bit of destructuring

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

@@ -201,6 +201,7 @@ describe('simple site', () => {
       frontMatter: {
         id: 'hello',
         title: 'Hello, World !',
+        sidebar_label: 'Hello sidebar_label',
       },
     });
   });
@@ -230,6 +231,7 @@ describe('simple site', () => {
       frontMatter: {
         id: 'hello',
         title: 'Hello, World !',
+        sidebar_label: 'Hello sidebar_label',
       },
     });
   });