docs(README): align the title font size

[chenjiahan]

Description

Align the title font size, use h2 for all sub-titles.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation.

[changeset-bot]

⚠️ No Changeset found

Latest commit: dee7cd6

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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