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

[slorber]

Motivation

Blog posts generally have assets such as images.
It makes sense to create a folder for a blog post and co-locate all images related to the blog post in its folder.
Example: 2021-05-12-announcing-docusaurus-two-beta/index.md or 2021/05/12/announcing-docusaurus-two-beta/index.md
See also: #5301

This PR makes:

• The default glob reading md files from subfolders
• Ensure that we extract a decent blog post date from the relative file path (including folders)
• Ensure that we can use relative images in Markdown frontmatter.

The following will now be supported and paths will be converted to Webpack file-loader require calls:

---
title: Announcing Docusaurus 2 Beta
authorImageURL: ./img/author.jpeg
image: ./img/social-card.png
---

Also:

• refactor a bit blog post-processing logic (no behavior change)
• content plugin guide docs linking to API reference doc
• generic mdx-loader system to convert frontmatter relative paths to require calls, and expose a frontMatterAssets export

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests + dogfooding on our own blog

Blog posts keep the same URL:

• https://docusaurus.io/blog/2021/05/12/announcing-docusaurus-two-beta
• https://deploy-preview-5309--docusaurus-2.netlify.app/blog/2021/05/12/announcing-docusaurus-two-beta

Social card + author img keeps working

The little blog page size increase is expected: it's the url-loader inlining a small image as base64 in the HTML file

[netlify]

✔️ [V2]

🔨 Explore the source changes: 031c2ef

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/610d533fe5a91a000810e4af

😎 Browse the preview: https://deploy-preview-5309--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	78
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5309--docusaurus-2.netlify.app/

[github-actions]

Size Change: +5.33 kB (+1%)

Total Size: 792 kB

Filename	Size	Change
website/build/blog/index.html	33.1 kB	+5.32 kB (+19%)	⚠️

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	36.1 kB	0 B
website/build/assets/css/styles.********.css	93.2 kB	0 B
website/build/assets/js/main.********.js	403 kB	+10 B (0%)
website/build/blog/2017/12/14/introducing-docusaurus/index.html	63.9 kB	0 B
website/build/docs/index.html	41.9 kB	-1 B (0%)
website/build/docs/installation/index.html	49.6 kB	-1 B (0%)
website/build/index.html	28.7 kB	0 B
website/build/tests/docs/index.html	22.6 kB	0 B
website/build/tests/docs/standalone/index.html	20.5 kB	0 B

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