docs: add graph execution example to API guide

[wunianze666-netizen]

Summary

This PR adds a small programmatic graph-execution section to the API guide.

Motivation

While reviewing the API docs, I noticed there is already good endpoint coverage, but it is still easy to get stuck on one key distinction: saved workflows are not executed directly, while enqueue_batch expects an executable graph. This came up in #8340, where a user could fetch workflows but was unsure how to actually run them through the HTTP API.

Changes

• add a short explanation of workflow vs executable graph
• add a minimal enqueue_batch example using a simple math graph
• document how to poll queue item status and where image outputs can be downloaded
• note that multi-user mode requires a Bearer token on these requests

Testing

• Not run: documentation-only change.

Notes

This is intended as a small and focused documentation improvement. I am happy to adjust the wording or scope if needed.

[Pfannkuchensack]

Isn't there a better place for this than in the MultiUser documentation?

[wunianze666-netizen]

Thanks, that makes sense. I moved the graph-execution example out of the Multi-User API docs into a dedicated development guide, and left a short pointer in the original page for multi-user readers who land there first.

[lstein]

Thank you for your contribution.

[lstein]

@wunianze666-netizen Could you rebase this on the main branch and push it back here? The branch will need to be up to date with main in order to merge.