Add a new lecture on job search

[jstac]

The code to be converted to JAX is here:

https://github.com/QuantEcon/book-dp1/blob/main/source_code_py/markov_js.py https://github.com/QuantEcon/book-dp1/blob/main/source_code_py/markov_js_with_sep.py

These can be combined into one lecture, with the second set of code as an exercise.

Background is in Section 3.3 of https://dp.quantecon.org/

[kp992]

Fixed in #172