docs: improve GET request example with custom headers explanation

[Nikita20010]

Summary

Improved the docs/user/quickstart.rst file by adding a clear example showing how to send custom headers with the requests.get() method.

Why

This enhances the documentation by demonstrating a common real-world use case — sending custom headers in HTTP GET requests.

Example Added

>>> import requests
>>> headers = {'User-Agent': 'CustomClient/1.0'}
>>> r = requests.get('https://api.github.com/events', headers=headers)

[sigmavirus24]

This is already documented