Add a new section titled "Linear Dynamics: Infinite Horizons"

[jstac]

This section could include, in the following order,

• https://intro.quantecon.org/geom_series.html
• https://intro.quantecon.org/eigen_I.html
• Tom's new lecture in the sandpit, currently at https://github.com/QuantEcon/python-lecture-sandpit.myst/blob/main/lectures/money_inflation_humphrey.md

At the same time, the section "Linear Dynamics" should become "Linear Dynamics: Finite Horizons"