Add support for processing issues from source-test-doc-upload

[Standard8]

The source-test-doc-upload builder can produce lots of errors and warnings such as:

[task 2026-01-08T12:00:09.746+00:00] TEST-KNOWN-FAIL | tools/tryselect/docs/tasks.rst4: | WARNING: undefined label: 'taskcluster task-graph generation'
[task 2026-01-08T12:00:09.746+00:00] TEST-UNEXPECTED-FAIL | browser/components/search/docs/index.rst2: | WARNING: Title underline too short.

Currently the code review bot says on the review comment:

The analysis task source-test-doc-upload failed, but we could not detect any defect.

We should be able to hook these up so that the errors and warnings are directly displayed on the files that caused them. This should help the errors be noticed more.

In bug 1890526, I've just posted a WIP patch for being able to export the error data to a json file, similar to what we have for mozlint builders.

It looks like we'll need to extend the doc upload task to process the output and display that on the review.