feat: global Markdown config option & frontMatterParser field

[Josh-Cena]

Motivation

Initial step to #4625! 🎉
Resolve #5568.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Unit tests: TODO
Dogfooded on website:

• Leading comments get removed: https://deploy-preview-5972--docusaurus-2.netlify.app/tests/pages/markdownPageTests
• Every blog post gets a tag: https://deploy-preview-5972--docusaurus-2.netlify.app/tests/blog

Caveats

The callback is called on every Markdown file, including partials. This leads to warnings about unused front matter if the hook injects extra front matter. We should find a way to: (a) call the custom callback which may do other useful stuff and (b) not trigger the warning if there was no front matter prior to the callback call.

Another possible solution I can think of is add another fileMetadata context value in addition to plugin, which includes createTime, author, fileName, etc. This may be useful also for the use-case of #5691.

[Josh-Cena]

I adopted a pretty inelegant solution to this problem by only checking for "apparent" front matter on partials. I don't know if we should pass in file paths (hard to implement) or just drop the warning for front matter in partials altogether

[netlify]

✔️ [V2]

🔨 Explore the source changes: 4394434

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/61cfd68ecd0d580007842432

😎 Browse the preview: https://deploy-preview-5972--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	71
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5972--docusaurus-2.netlify.app/

[github-actions]

Size Change: +606 B (0%)

Total Size: 670 kB

Filename	Size	Change
website/build/assets/js/main.********.js	498 kB	+606 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	40.1 kB
website/build/assets/css/styles.********.css	102 kB
website/build/index.html	29.6 kB

_{compressed-size-action}

[Josh-Cena]

Closing until #6370. That one would mean totally turning this over, so I'd rather implement that first.