docs: mention equivalent config syntaxes

[thadguidry]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Fixes part of #8925

Test Plan

Test links

https://deploy-preview-8951--docusaurus-2.netlify.app/docs/configuration/#syntax-to-declare-docusaurus-config

https://deploy-preview-8951--docusaurus-2.netlify.app/docs/api/docusaurus-config/

Related issues/PRs

#8925

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
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[thadguidry]

To me, this now looks and feels really good. The missing info and examples that would have simply explained things for me at the very beginning of my confusion and opening issue #8925. I hope it now matches all our expectations for more clear docs for novice users trying out Docusaurus for the first time.

@slorber so, what do you think now?

[slorber]

Thanks, will do some minor edits and merge