Add style check for emphasis and definitions usage

[mmcky]

Summary

Add a style check to ensure consistent usage of emphasis and definitions according to the QuantEcon style guide.

Style Guide Reference

https://manual.quantecon.org/styleguide/writing.html\#emphasis-and-definitions

Rules

• Bold () should be used for definitions
• Italic () should be used for emphasis

Implementation

We should implement an automated check (linter or CI job) that verifies:

1. Terms being defined for the first time use bold formatting
2. Emphasis uses italic formatting
3. These conventions are applied consistently across all lecture files

Files to Check

All markdown files in the lectures/ directory (~80+ lecture files)

Benefits

• Ensures consistency across all lectures
• Helps maintain professional documentation standards
• Makes it easier for readers to identify new terms vs. emphasized text