GH-113425: Add example for default_namespace in ElementTree docs

[Rohithdgrr]

Summary

Adds an example to the ElementTree documentation showing how to use the default_namespace parameter to preserve default namespaces during XML serialization.

Problem

When serializing XML that contains a default namespace, ElementTree adds a generated ns0: prefix instead of preserving the default namespace. This behavior is documented but lacks a clear example showing the workaround.

Solution

Added a new subsection "Serializing XML with Default Namespace" in the Tutorial section with:

• Example showing the problem (ns0 prefix appearing)
• Example showing the solution (using default_namespace parameter)

Issue

Fixes #113425

Checklist

• Documentation changes in Doc/library/xml.etree.elementtree.rst
• Includes code examples with expected output
• References the relevant parameter default_namespace added in 3.8

• Issue: Add example to ElementTree documentation for default namespace (to avoid spurious ns0: namespace prefix) #113425

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📚 Documentation preview 📚: https://cpython-previews--147976.org.readthedocs.build/

[merwok]

Please keep the target for the following section

[merwok]

I don’t agree with the phrasing here! (after instead of)

[merwok]

Please disclose if you use chatbots to assist contribution. (I have a suspicion because of the way the PR message is structured.)