feat: blog posts support /YYYY/MM/DD/blog-post/index.md pattern + blog frontmatter can reference relative images (#5309)

* POC of blog post folder

* add parseBlogFileName with tests + refactor and extract processBlogSourceFile in separate method

* improve blog date pattern doc + link from content plugin guides to API ref docs

* Some FrontMatter fields should be able to reference relative image assets, converted to Webpack require calls and exposed as frontMatterAssets

* remove log

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

@@ -467,6 +467,12 @@ export default function pluginContentBlog(
                     // For blog posts a title in markdown is always removed
                     // Blog posts title are rendered separately
                     removeContentTitle: true,
+                    // those frontMatter fields will be exported as "frontMatterAssets" and eventually be converted to require() calls for relative file paths
+                    frontMatterAssetKeys: [
+                      'image',
+                      'authorImageURL',
+                      'author_image_URL',
+                    ],
                   },
                 },
                 {