Add GXF sample file with subtitles for testing

[mohithshuka]

• Added sample_with_subtitles.gxf (SMPTE 360M compliant)
• Added generator script create_gxf_sample.py
• Added documentation README.md
• Sample contains CEA-608 closed captions
• Addresses issue [REQUEST]GXF Sample #1748

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{pull request content here}

[mohithshuka]

Hi! I've added a GXF sample file with embedded CEA-608 subtitles for testing CCExtractor.

Files included:

• sample_with_subtitles.gxf - A valid SMPTE 360M compliant GXF file (247 KB)
• create_gxf_sample.py - Python script to generate the sample
• README.md - Complete documentation

Sample specifications:

• Format: GXF (SMPTE 360M)
• Subtitle format: CEA-608 closed captions
• Caption text: "Hello, World!"
• Duration: ~2 seconds

The sample can be tested with:
ccextractor docs/samples/gxf/sample_with_subtitles.gxf -o output.srt

This addresses issue #1748. Please let me know if you need any modifications or additional samples!

[cfsmp3]

Closing this PR due to issues:

1. Empty PR description: No explanation of what this PR does or why it's needed.

2. Binary file in repo: Adding binary sample files (.gxf) directly to the repository is not the right approach. Sample files should be hosted on the CCExtractor sample platform (https://sampleplatform.ccextractor.org/), not committed to the git repository.

3. Generated content: The PR includes a Python script to "create" a GXF sample - if the sample is generated/synthetic, it may not represent real-world GXF files with subtitles.

If you have real GXF sample files with subtitles, please upload them to the sample platform instead.