ci: prepare editable docling-parse for docs

[geoHeil]

Summary

• detect docs builds that use an editable ../docling-parse source
• clone docling-project/docling-parse into the expected sibling path before uv sync
• keep the existing frozen uv install behavior intact

Verification

• inspected failing docs job 73611235441 from run 25118188355
• reproduced the missing ../docling-parse install failure on PR feat: add threaded docling-parse (v6) PDF backend  #3377 merge
• verified uv sync --frozen --all-extras --all-packages --dry-run succeeds after the sibling checkout exists
• parsed .github/workflows/docs.yml with PyYAML
• git diff --check

[github-actions]

✅ DCO Check Passed

Thanks @geoHeil, all your commits are properly signed off. 🎉

[mergify]

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[geoHeil]

closing as in agreement with @dolfim-ibm this is not a functionality we want to support right now