Fix confusing variable names in collection example docs

[Copilot]

The collection creation example shadowed the imported sigmf module by reusing it as a variable name, and created streams/sigmf variables that were never used. The get_stream_names() call was also isolated from any meaningful context.

Changes

• Removed the unused streams and sigmf assignments from the collection creation block
• Moved get_stream_names() usage into the loading example, where it's more illustrative
• Renamed the list comprehension variable to all_sigmffiles with explicit stream_name= kwarg

# Before (in creation block — unused and shadows import)
streams = collection.get_stream_names()
sigmf = [collection.get_SigMFFile(stream) for stream in streams]

# After (in loading block — shows practical usage)
all_sigmffiles = [collection.get_SigMFFile(stream_name=stream_name)
                  for stream_name in collection.get_stream_names()]

[Copilot]

Pull request overview

Updates the advanced documentation’s SigMF Collection example to avoid shadowing the sigmf module and to place stream enumeration in a more relevant “loading” context.

Changes:

• Removed unused streams / sigmf assignments from the collection creation example.
• Added an example that enumerates stream names and loads all streams’ SigMFFile objects when loading a collection.
• Updated the get_SigMFFile call to use the explicit stream_name= keyword in the new comprehension.

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