docs: fix Contribute node import error

[ubansi]

When I followed the Contribute Node documentation, I encountered an import error. This commit fixes the error, which will help reduce debugging time for all future contributors.

Summary

• Fixes import path: Changed import path from invokeai.app.invocations.baseinvocation to the correct invokeai.invocation_api.
• Cleans up imports: Removed unused imports (ResourceOrigin, ImageCategory) from the example code.
• Updates example node: Changed the example import class in __init__.py from CoolInvocation to ResizeInvocation (to match the documented example node). This ensures that following the documented steps now results in a working node.
• May future new contributors never know the specific flavor of import error pain this caused.

Related Issues / Discussions

Addresses a common issue encountered by users attempting to develop custom nodes, specifically related to import errors when following the documentation.

QA Instructions

To verify this fix, please follow the steps outlined in the "Contribute Nodes" documentation section that was modified by this PR. Confirm that the import error previously encountered no longer occurs when setting up a basic custom node as described in the updated documentation.

Merge Plan

Standard merge is sufficient for this documentation change.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) (Automated tests are typically not required for documentation changes)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR) (Documentation fixes usually don't require a What's New entry)

[hipsterusername]

Thank you!