[linear_equations] Linea Algebra Lecture improvements

[HumphreyYang]

This issue will collect comments from the Feb 2024 reading group meeting on
https://intro.quantecon.org/linear_equations.html

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

• Give an overview that states the aim of the lecture -- to learn how to solve large systems of linear equations, and add a link to https://intro.quantecon.org/linear_equations.html#more-goods
• Add a link to numpy lecture.
• Potentially break the lecture into two -- one on concepts and one on computation.
• Potentially add a lecture on matrix calculus (perhaps using sympy matrix calculus?)