Fix sidebar when figure within admointion

[Tom-van-Woudenberg]

Some pages render another secondary sidebar without the expected id. The previous id-only lookup could miss it, trigger fallback insertion in the content column, and cause the “extra inner margin” effect.

Now the script:

1. Detects an existing secondary sidebar by either id or class.
2. Uses that real sidebar for sticky items instead of creating a fallback in the content column.
3. Only creates a fallback when no secondary sidebar exists at all, and gives it the expected id for compatibility.