[rational_expectations] Update content and styling

[xuanguang-li]

This pull request makes a series of stylistic and clarity improvements to the lectures/rational_expectations.md file. There are no code changes, as the control problem is solved using quantecon.LQ, for which numpy works well.

Key changes:

Mathematical and technical clarifications:

• Clarified the meaning of $Y_{t+1}$ in different places.
• Updated the Euler equation to use the belief function $H$ consistently, and added an equivalent form for clarity.
• Added an explanation for the failure of contractivity using Blackwell's condition, improving conceptual understanding.
• Clarified and corrected the law of motion for the case of a unit measure of firms in the first exercise.

Formatting and reference improvements:

• Converted links to internal documentation references
• Added a proper definition block.
• Changed all section and subsection headings to sentence case

[github-actions]

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📚 Changed Lecture Pages: rational_expectations