Remove kernel builder pattern for Python: sk.kernel_create() -> sk.Kernel()

[awharrison-28]

Motivation and Context

This PR makes creating a kernel instance more pythonic. The builder pattern is removed and the kernel is constructed using Kernel(...).

Work originally started with #137

Description

• Removed kernel_builder.py.
• Instance creation pattern kernel = sk.create_kernel() -> kernel = sk.Kernel()
• For notebook 6, there is no need to create a separate kernel for the GitHub files. Changed to use a separate collection.

Contribution Checklist

• [x] The code builds clean without any errors or warnings
• [x] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [x] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [x] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile:

[dluc]

@mkarle I'll merge after your ✅