Add design considerations

[lognaturel]

Closes #10

What has been done to verify that this works as intended?

This mostly inserts the notes from the issue. It feels like they've stood up well through the functionality we've added recently!

Why is this the best possible solution? Were any other approaches considered?

I feel like this matches what we've been considering as part of API design but it's only good if @lindsay-stevens agrees!

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

None.

Do we need any specific form for testing your changes? If so, please attach one.

No.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

It is!

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments