feat(v2): various markdown string parsing improvements/fixes

[slorber]

Motivation

Refactors and improvements of the markdown parsing system:

• Extract markdown-related code + tests in dedicated file
• Clearer API surface
• Title found in markdown content (not frontmatter) is returned as a separate attribute, not added to frontMatter.title => ability to handle those cases separately
• Ability to extract title from markdown documents using the alternate markdown syntax (title\n===)
• Fix empty excerpt when Markdown doc has a title: excerpt should simply skip/ignore the title
• Duplicate title warning: better wording + ability to disable it (as it can be annoying for some sites after upgrading, cf RN website)
• Initial code to have more strict frontmatter validation using Joi (to be completed)

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

Related PRs

Follow-up of #4485 cc @armano2

Also related to #4582 (as we want to support better sites without any frontmatter at all)

[netlify]

[V1]

Built with commit ee11036

https://deploy-preview-4590--docusaurus-1.netlify.app

[netlify]

[V2]

Built with commit ee11036

https://deploy-preview-4590--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	75
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4590--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 576 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.2 kB	0 B
website/build/assets/js/main.********.js	401 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	61.1 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	26.1 kB	0 B

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