fix(v2): use frontmatter title at first for paginated links (#4859)

packages/docusaurus-plugin-content-docs/src/__tests__/__fixtures__/simple-site/docs/foo/baz.md

@@ -4,6 +4,8 @@ title: baz
 slug: bazSlug.html
 ---
 
+# Baz markdown title
+
 ## Images
 
 Like links, Images also have a footnote style syntax

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

@@ -190,7 +190,7 @@ Object {
   \\"unversionedId\\": \\"foo/baz\\",
   \\"id\\": \\"foo/baz\\",
   \\"isDocsHomePage\\": false,
-  \\"title\\": \\"baz\\",
+  \\"title\\": \\"Baz markdown title\\",
   \\"description\\": \\"Images\\",
   \\"source\\": \\"@site/docs/foo/baz.md\\",
   \\"sourceDirName\\": \\"foo\\",
@@ -204,7 +204,7 @@ Object {
   },
   \\"sidebar\\": \\"docs\\",
   \\"previous\\": {
-    \\"title\\": \\"Remarkable\\",
+    \\"title\\": \\"Bar\\",
     \\"permalink\\": \\"/docs/foo/bar\\"
   },
   \\"next\\": {
@@ -423,7 +423,7 @@ Object {
               },
               {
                 \\"type\\": \\"link\\",
-                \\"label\\": \\"baz\\",
+                \\"label\\": \\"Baz markdown title\\",
                 \\"href\\": \\"/docs/foo/bazSlug.html\\"
               }
             ]

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

@@ -286,7 +286,7 @@ describe('simple site', () => {
       isDocsHomePage: false,
       permalink: '/docs/foo/bazSlug.html',
       slug: '/foo/bazSlug.html',
-      title: 'baz',
+      title: 'Baz markdown title',
       editUrl:
         'https://github.com/facebook/docusaurus/edit/master/website/docs/foo/baz.md',
       description: 'Images',
@@ -345,7 +345,7 @@ describe('simple site', () => {
       isDocsHomePage: false,
       permalink: '/docs/foo/bazSlug.html',
       slug: '/foo/bazSlug.html',
-      title: 'baz',
+      title: 'Baz markdown title',
       editUrl: hardcodedEditUrl,
       description: 'Images',
       frontMatter: {

packages/docusaurus-plugin-content-docs/src/index.ts

@@ -199,7 +199,11 @@ export default function pluginContentDocs(
             nextId,
           } = sidebarsUtils.getDocNavigation(doc.id);
           const toDocNavLink = (navDocId: string): DocNavLink => ({
-            title: docsBaseById[navDocId].title,
+            // Use frontMatter.title in priority over a potential # title found in markdown
+            // See https://github.com/facebook/docusaurus/issues/4665#issuecomment-825831367
+            title:
+              docsBaseById[navDocId].frontMatter.title ||
+              docsBaseById[navDocId].title,
             permalink: docsBaseById[navDocId].permalink,
           });
           return {