Replace MathML with LaTeX syntax in python_by_example.md

[mmcky]

Summary

This PR replaces verbose MathML syntax with clean LaTeX $...$ notation throughout python_by_example.md.

Problem

The file contained 24 MathML blocks like this:

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <msub>
    <mi>ϵ</mi>
    <mi>t</mi>
  </msub>
</math>

These are verbose, hard to read, and inconsistent with the bulk translations which use LaTeX.

Solution

All MathML blocks have been converted to clean LaTeX:

• $\epsilon_t$ instead of 7-line MathML
• $(0, 1)^2$ instead of 11-line MathML
• $b_{t+1} = (1 + r) b_t$ instead of 20-line MathML

Changes

• 24 MathML blocks converted to LaTeX
• 210 lines removed, 34 lines added
• File is now consistent with bulk translation style
• Math expressions are now readable inline

Examples of Conversions

Location	Before (MathML)	After (LaTeX)
White noise	18-line block	$\epsilon_0, \epsilon_1, \ldots, \epsilon_T$
Axis labels	7-line block	$t$ and $\epsilon_t$
Bank balance	20-line block	$b_{t+1} = (1 + r) b_t$
Monte Carlo	11-line block	$(0, 1)^2$
Pi estimation	Multiple blocks	$\pi$, $A$, $r=1/2$

Review Notes

• No content changes - only formatting of math expressions
• All LaTeX syntax matches the bulk translation version
• File should render correctly in Jupyter Book

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[Zahra-khanzadeh]

Hi @mmcky ,

Could you please add the indexes for this lecture in the same style as the Getting Started lecture? Once that's done, we can go ahead and merge.

Thanks a lot!