docs: add mention that themeConfig.footer.copyright supports html strings

[fxpby]

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

I want to configure copyright text to have some links, and via packages/docusaurus-theme-classic/src/theme/Footer/Copyright/index.tsx knew copyright supports the custom HTML, hope to add this description for docs.
And I had checked copyright alreadly supports the custom HTML in old version.

Test Plan

Test links

Deploy preview: https://deploy-preview-8761--docusaurus-2.netlify.app/

Related issues/PRs

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	f4e3ac5
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/640e08a283d02800081d6bb9
😎 Deploy Preview	https://deploy-preview-8761--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 90	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 84	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

LGTM thanks 👍