Doc: Add exponentiation example to tutorial introduction

[TheAhumMaitra]

This PR updates the tutorial's introduction section by adding an example of the
** operator (exponentiation). It demonstrates how to raise a number to a power,
using 2 ** 9 as the example for clarity.

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📚 Documentation preview 📚: https://cpython-previews--138096.org.readthedocs.build/

[python-cla-bot]

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[StanFromIreland]

There already are examples, a paragraph below, when the operator is introduced:

With Python, it is possible to use the ** operator to calculate powers [#]_::

 >>> 5 ** 2  # 5 squared
 25
 >>> 2 ** 7  # 2 to the power of 7
 128

It is not in the section you have added the example to as it has not yet introduced exponentiation:

... the operators +, -, * and / ...

[ZeroIntensity]

Yeah, sorry. I agree with Stan -- we're only introducing the arithmetic operators here. ** is introduced right below.