feat(core): async docusaurus.config.js creator function

[slorber]

Motivation

Fixes #3791

We should be able to use async code to create the config.

Later: we'll try to also support ESM config + top-level await

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

tests + dogfood

[netlify]

✔️ [V2]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	85
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-6165--docusaurus-2.netlify.app/

[Josh-Cena]

This actually means we can support ESM remark plugins, because they can now be await import()'ed. We can upgrade the docs on this

[github-actions]

Size Change: 0 B

Total Size: 662 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	40.1 kB
website/build/assets/css/styles.********.css	102 kB
website/build/assets/js/main.********.js	490 kB
website/build/index.html	29.6 kB

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[slorber]

This actually means we can support ESM remark plugins, because they can now be await import()'ed. We can upgrade the docs on this

Maybe we should dogfood this first and ensure it works before documenting it?

I'm not sure to see how it would actually work considering we are still using Remark 12 and most ES plugins are for Remark 13? It is retro compatible enough so that Remark 13 plugins can work with our current setup?

[Josh-Cena]

Hmmm, that part I don't know😄 We can still test