Add gold and s&p500 volatility for comparison

[golmit]

Description

• This change includes an updated version of the annualized volatility graph, using gold and the S&P 500 as reference points.
• Changes the notation for the monetary growth rate from m_{t+1} to g_{t+1}.

Type of Change

• Minor feature (Small changes to slides or other files)

Checklist:

• I have performed a self-review of my own changes
• I have made sure that any changed LaTeX files compile properly
• I have assigned at least one reviewer