chore: check MDX even by VS Code cspell extension

[tats-u]

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

Typos in MDX files are not checked in VS Code even if we have installed the cspell extension. It works only for JS/TS and vanilla Markdowns.

Test Plan

1. Install and enable the cspell extension for VS Code
2. Open README.md in VS Code and add a typo there (still warned & don't save and just discard the change)
3. Open some other MDX files and add a typo there (now warned & don't save and just discard the change)

Test links

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

Note: content has not been changed

Related issues/PRs

streetsidesoftware/vscode-spell-checker#657

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	78b3de8
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/659abb6a2c9400000856c57b
😎 Deploy Preview	https://deploy-preview-9713--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 configuration.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟢 94	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/installation	🟠 69	🟢 98	🟢 100	🟢 100	🟠 89	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog	🟠 70	🟢 100	🟢 100	🟢 90	🟠 89	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 97	🟢 100	🟢 100	🟠 89	Report
/blog/tags/release	🟠 70	🟢 100	🟢 100	🟠 80	🟠 89	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟢 90	🟠 89	Report

[slorber]

thanks 👍