[business_cycles] Improvement suggestions

[HumphreyYang]

This issue collects minor code and style comments from the Feb 2024 reading group meeting on
https://intro.quantecon.org/business_cycle.html

Many thanks @SylviaZhaooo and @longye-tian for suggestions.

• Code in section #gdp-growth-rate repeats the first section where it teaches readers how to retrieve data.
• Add a section on what is business cycles and what defines them.
• Add a table of contents at the beginning of lectures.
• consumer_sentiment -> production_output.