new lecture on social mobility

[jstac]

Consider adding a lecture on social mobility

• https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2889760

https://drive.google.com/file/d/19VdI51WJXqYLWJSh44ARG_gjFgob0NSb/view

[jstac]

Shifting this to the intro lecture series, where the Markov chain prerequisites already sit.

Drafted as "Measuring Mobility" in QuantEcon/lecture-python-intro#839 — mobility matrices over wealth quantiles, four measures (Shorrocks, Bartholomew, second eigenvalue, mean first passage time), applied to toy chains and to PSID wealth transition matrices.

Closing here; follow the intro-series PR instead.