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/__tests__/blogUtils.test.ts

@@ -0,0 +1,94 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {parseBlogFileName} from '../blogUtils';
+
+describe('parseBlogFileName', () => {
+  test('parse file', () => {
+    expect(parseBlogFileName('some-post.md')).toEqual({
+      date: undefined,
+      text: 'some-post',
+      slug: '/some-post',
+    });
+  });
+
+  test('parse folder', () => {
+    expect(parseBlogFileName('some-post/index.md')).toEqual({
+      date: undefined,
+      text: 'some-post',
+      slug: '/some-post',
+    });
+  });
+
+  test('parse nested file', () => {
+    expect(parseBlogFileName('some-post/some-file.md')).toEqual({
+      date: undefined,
+      text: 'some-post/some-file',
+      slug: '/some-post/some-file',
+    });
+  });
+
+  test('parse nested folder', () => {
+    expect(parseBlogFileName('some-post/some-subfolder/index.md')).toEqual({
+      date: undefined,
+      text: 'some-post/some-subfolder',
+      slug: '/some-post/some-subfolder',
+    });
+  });
+
+  test('parse file respecting date convention', () => {
+    expect(
+      parseBlogFileName('2021-05-12-announcing-docusaurus-two-beta.md'),
+    ).toEqual({
+      date: new Date('2021-05-12Z'),
+      text: 'announcing-docusaurus-two-beta',
+      slug: '/2021/05/12/announcing-docusaurus-two-beta',
+    });
+  });
+
+  test('parse folder name respecting date convention', () => {
+    expect(
+      parseBlogFileName('2021-05-12-announcing-docusaurus-two-beta/index.md'),
+    ).toEqual({
+      date: new Date('2021-05-12Z'),
+      text: 'announcing-docusaurus-two-beta',
+      slug: '/2021/05/12/announcing-docusaurus-two-beta',
+    });
+  });
+
+  test('parse folder tree respecting date convention', () => {
+    expect(
+      parseBlogFileName('2021/05/12/announcing-docusaurus-two-beta/index.md'),
+    ).toEqual({
+      date: new Date('2021-05-12Z'),
+      text: 'announcing-docusaurus-two-beta',
+      slug: '/2021/05/12/announcing-docusaurus-two-beta',
+    });
+  });
+
+  test('parse folder name/tree (mixed) respecting date convention', () => {
+    expect(
+      parseBlogFileName('2021/05-12-announcing-docusaurus-two-beta/index.md'),
+    ).toEqual({
+      date: new Date('2021-05-12Z'),
+      text: 'announcing-docusaurus-two-beta',
+      slug: '/2021/05/12/announcing-docusaurus-two-beta',
+    });
+  });
+
+  test('parse nested folder tree respecting date convention', () => {
+    expect(
+      parseBlogFileName(
+        '2021/05/12/announcing-docusaurus-two-beta/subfolder/subfile.md',
+      ),
+    ).toEqual({
+      date: new Date('2021-05-12Z'),
+      text: 'announcing-docusaurus-two-beta/subfolder/subfile',
+      slug: '/2021/05/12/announcing-docusaurus-two-beta/subfolder/subfile',
+    });
+  });
+});