fix(v2): fix title logic (meta vs heading) + ignore fixed anchor id syntax

[slorber]

Motivation

Fixes #4665 (comment) + handle fixed anchor syntax

Keep behavior we had before:

• frontmatter.title controls page meta in priority
• markdown # title controls page h1 heading in priority

We should rather not use fixed anchor ids syntax on top level h1 headings, as user should not use h1 headings in his doc normally anyway:

• writeHeadingIds cli will not write custom anchor ids for h1 md titles, and # myTitle remains untouched
• a markdown title as # myTitle {#my-anchor} will now parse correctly title = myTitle

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

test

[netlify]

[V1]

Built with commit 5354b3a

https://deploy-preview-4688--docusaurus-1.netlify.app

[netlify]

[V2]

Built with commit 5354b3a

https://deploy-preview-4688--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	87
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4688--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 631 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	88 kB	0 B
website/build/assets/js/main.********.js	453 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	27.1 kB	0 B

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