chore: reorganize docs image assets

[HynLcc]

Summary

• reorganize active API, selfhosted, and changelog images under images/{top-level-group}/{topic}
• move currently unreferenced image assets into images/orphans/2026-05-29 instead of deleting them
• standardize API/selfhosted screenshot markup and fill changelog image alt text without applying docs-screenshot to changelog media

Scope guard

• rebuilt from current origin/main and cherry-picked only the image asset commits
• excludes unrelated app-builder and root changelog changes from the older branch base

Checks

• custom image reference audit: missing=0, unrefActive=0, refsToOrphans=0
• custom image markup audit: standardViolations=0, changelogEmptyAlt=0, changelogDocsScreenshot=0
• git diff --check origin/main...HEAD
• mintlify validate