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

website/docs/guides/creating-pages.md

@@ -2,6 +2,7 @@
 id: creating-pages
 title: Creating Pages
 slug: /creating-pages
+sidebar_label: Pages
 ---
 
 In this section, we will learn about creating pages in Docusaurus.
@@ -18,6 +19,12 @@ Pages do not have sidebars, only [docs](./docs/docs-introduction.md) do.
 
 :::
 
+:::info
+
+Check the [Pages Plugin API Reference documentation](./../api/plugins/plugin-content-pages.md) for an exhaustive list of options.
+
+:::
+
 ## Add a React page {#add-a-react-page}
 
 Create a file `/src/pages/helloReact.js`: