fix: run generated docs check on docs-only PRs

[adp2201]

Which issue does this PR close?

Closes #19095

Rationale for this change

The generated-doc drift check currently lives in rust.yml, but rust.yml ignores docs-only and markdown-only PRs via paths-ignore:

• docs/**
• **.md

That means a PR can directly edit autogenerated files such as:

• docs/source/user-guide/configs.md
• docs/source/user-guide/sql/aggregate_functions.md
• docs/source/user-guide/sql/scalar_functions.md
• docs/source/user-guide/sql/window_functions.md

and still pass CI, because the workflow that runs:

• ./dev/update_config_docs.sh
• ./dev/update_function_docs.sh

never executes.

I reproduced this locally and summarized the findings on the issue:

• Run config_docs CI check is not run on PRs to only change generated docs (resulting in CI failures) #19095 (comment)

What changes are included in this PR?

1. Add a dedicated Generated Docs workflow in .github/workflows/docs_generated.yaml
2. Trigger that workflow on:
   • generated doc files
   • generator scripts
   • ci/scripts/utils/tool_versions.sh
   • datafusion/**
   • the workflow file and setup actions it depends on
3. Preserve the existing job name:
   • check configs.md and ***_functions.md is up-to-date
4. Remove the old generated-doc check job from rust.yml

This keeps rust.yml path filtering unchanged while ensuring docs-only edits to autogenerated files still run the drift check.

Are these changes tested?

Yes, locally:

• Parsed workflow YAML successfully with PyYAML
• Verified trigger behavior for:
  • generated docs edits
  • ordinary docs edits
  • source/script/setup changes
• Ran:
  • ./dev/update_config_docs.sh
  • ./dev/update_function_docs.sh
• Confirmed both commands exited successfully and produced no diffs in generated docs

Are there any user-facing changes?

No. This is CI/process-only.