feat(v2): add option for blog meta title

[DeadEnglish]

Motivation

Fixing the issue raised within #3542

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work. Bonus points for screenshots and videos!)

• run yarn start
• navigate to 'blog' page
• check the title has been updated with blog title rather than previous blog
• stop project from running
• change the blog title within the blog config
• restart docusarus
• navigate to 'blog' page
• check blog title has changed

Related PRs

#3542

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[docusaurus-bot]

Deploy preview for docusaurus-2 ready!

Built with commit d66064a

https://deploy-preview-3571--docusaurus-2.netlify.app

[DeadEnglish]

CLA has been signed but I don't seem to be able to re-run the check 😬

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[lex111]

Seems good.

[slorber]

LGTM, will just change the default title to "Blog" instead of "Blog title" for retrocompat, as it would be applied to our own blog 🤪

https://deploy-preview-3571--docusaurus-2.netlify.app/classic/blog