st:include page reference doesn't seem to be clickable even though `JellyReferenceContributor` supposed to handle it

### Jenkins and plugins versions report

IntelliJ Ultimate 2021.3.2

### What Operating System are you using (both controller, and any agents involved in the problem)?

macOS

### Reproduction steps

1. Open a Jelly file with `` tag. No `it` or `form` tag.
2. Try to click on the page reference in the attribute value

### Expected Results

IntelliJ navigates to the file

### Actual Results

The attribute value is not a reference.

### Anything else?

_No response_

Start by inspecting JellyReferenceContributor and reproducing the issue in a Jelly file containing an st:include tag with a page attribute but no it or form tag. Confirm that the page attribute is treated as a reference and that clicking it navigates to the referenced file.

Written by the indexing model from the issue text.