Checked-in design docs

Consider setting up a process for design proposals via checked-in Markdown files which are reviewed just like source code.

For inspiration / reference, see:

• .NET designs
• C# language design
• Kubernetes enhancements
• PyTorch RFCs

Checked in design docs should include snippets of code that would work after the design is implemented:

from modern_python_package import inner_method

big_number = 42

cool_variable = inner_method("short string", big_number)

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