[SPARK-53666][INFRA][FOLLOWUP] Use absolute output path for generate-llms-txt.py

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to use an absolute output path ("$SELF/_site/llms.txt") instead of a relative path (_site/llms.txt) when dev/create-release/release-build.sh uses generate-llms-txt.py.

spark/dev/create-release/release-build.sh

Lines 839 to 841 in 47a8d2a

	python "$SELF/generate-llms-txt.py" \
	--docs-path . \
	--output _site/llms.txt \

Why are the changes needed?

Currently, Apache Spark release CI has been broken. (https://github.com/apache/spark/actions/workflows/release.yml)
This is a release blocker for Apache Spark 4.1.0.

• https://github.com/apache/spark/actions/runs/19060666797 (2025-11-03)

+ echo 'Generating llms.txt...'
Generating llms.txt...
+ python /opt/spark-rm/generate-llms-txt.py --docs-path . --output _site/llms.txt --version 4.2.0-SNAPSHOT
Traceback (most recent call last):
  File "/opt/spark-rm/generate-llms-txt.py", line 206, in <module>
    main()
  File "/opt/spark-rm/generate-llms-txt.py", line 202, in main
    generate_llms_txt(docs_path, output_path, args.version)
  File "/opt/spark-rm/generate-llms-txt.py", line 159, in generate_llms_txt
    with open(output_path, "w", encoding="utf-8") as f:
FileNotFoundError: [Errno 2] No such file or directory: '/_site/llms.txt'

• https://github.com/apache/spark/actions/runs/19094119034 (2025-11-04)

+ echo 'Generating llms.txt...'
Generating llms.txt...
+ python /opt/spark-rm/generate-llms-txt.py --docs-path . --output _site/llms.txt --version 4.2.0-SNAPSHOT
Traceback (most recent call last):
  File "/opt/spark-rm/generate-llms-txt.py", line 206, in <module>
    main()
  File "/opt/spark-rm/generate-llms-txt.py", line 202, in main
    generate_llms_txt(docs_path, output_path, args.version)
  File "/opt/spark-rm/generate-llms-txt.py", line 159, in generate_llms_txt
    with open(output_path, "w", encoding="utf-8") as f:
FileNotFoundError: [Errno 2] No such file or directory: '/_site/llms.txt'

Does this PR introduce any user-facing change?

No behavior change because this is a release script.

How was this patch tested?

Manual test.

$ mkdir /tmp/SPARK-53666
$ dev/create-release/do-release-docker.sh -t SPARK-53666 -d /tmp/SPARK-53666 -n -s docs
...
= Building spark-rm image with tag SPARK-53666...
...

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Thank you, @zhengruifeng .

[dongjoon-hyun]

Let me merge this and run the CI.

[allisonwang-db]

Thanks for the fix!