docs: add API example for image promotion between branches

[forfoxssake]

Summary

• Adds a new "Promoting an image to another branch" section to the API usage examples page
• Documents the two-step workflow: finding an image's internal name via fieldSelector, then creating an ImagePromotion resource to promote it to a target branch

Notes

• API-only for now; UI documentation will follow in a separate PR
• Links to the relevant API reference endpoints (listImage, createImagePromotion)

[Copilot]

Pull request overview

Adds documentation for promoting a container image from one branch to another via the REST API, describing the required lookup + promotion workflow.

Changes:

• Adds a new “Promoting an image to another branch” section with a two-step workflow.
• Provides curl examples for listing/filtering images and creating an ImagePromotion.
• Adds a variables table and links to the relevant API reference endpoints.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[Brendan Le Glaunec (Ullaakut)]

Just a nitpick, and a question about whether the new sentence is factually correct. If it is, it means the previous docs were plain false.