docs: add theme-common to API docs

[Josh-Cena]

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

theme-common is in a very, very awkward situation. Its APIs are not meant to be public, but are more or less so through ejecting theme components. Nevertheless, there are some very stable APIs that are worth documenting.

For now, I've simply moved the useColorMode hook documentation here; in the near future, we can include more utilities that are stable. For example, usePluralForm is also mentioned in the i18n tutorial and worth including in the future.

@slorber Before the official release, we need to decide the subset of theme-common APIs to be made stable and documented. Those that rely on unstable theme-common APIs would not be safe for eject. This is related to #6116

I even think some hooks like usePrevious and ReactContextError can be in core because they may be useful to every user and plugin...

Test Plan

Test links

Deploy preview: https://deploy-preview-7382--docusaurus-2.netlify.app/docs/api/misc/@docusaurus/theme-common/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 65	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟢 91	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +607 B (0%)

Total Size: 811 kB

Filename	Size	Change
website/.docusaurus/globalData.json	52.4 kB	+175 B (0%)
website/build/assets/js/main.********.js	615 kB	+432 B (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	105 kB
website/build/index.html	38.8 kB

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