Reacting to Input with State docs

[parmeyjohn]

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Issues

#863 Rewriting the docs for the 'Reacting to Input with State' section under 'Managing State' located at: https://reactive-python.github.io/reactpy/learn/reacting-to-input-with-state/

Solution

The examples and descriptions have been refactored from JavaScript/ReactJS to ReactPy code and terminology.

Checklist

• [ ] Tests have been included for all bug fixes or added functionality.
• [ ] The changelog.rst has been updated with any significant changes.