Add spellcheck feature to editor documentation

[mintlify-development]

Summary

• Added built-in spellcheck as a key feature in the web editor overview
• Documented that visual mode includes browser-native spellcheck in the pages guide

This reflects the recent change in mintlify/mint@4d0e57c that enables spellcheck in the visual editor.

---

Note

Low Risk
Low risk documentation-only change that adds mention of browser-native spellcheck; no product logic or data handling is modified.

Overview
Updates the web editor docs to highlight built-in spellcheck: adds it to the editor/index.mdx key features list and notes in editor/pages.mdx that Visual mode uses browser-native spellcheck to catch typos while writing.

^{Written by Cursor Bugbot for commit fdb8b81. This will update automatically on new commits. Configure here.}