Update how to use API docs

[lognaturel]

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

Ran it, verified image and link.

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

There's probably a way to make the image more attractive but this is at least more useful.

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, docs-only.

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 an update to docs!

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