[SPARK-57938][DOCS] Print active SKIP_ flags when Ruby doc build script runs

[nchammas]

What changes were proposed in this pull request?

It's useful to know what skip flags are active when the documentation build runs. Instead of echo-ing them in the places where we want to know, the build script now prints them itself when it runs.

Extracted from #57010.

Why are the changes needed?

Helps troubleshoot builds when we want to know what exactly was supposed to run.

The new method to print flags is naive in that it prints all flags that begin with SKIP_, even if they have no special meaning to the script. It's a minor tradeoff that lets us avoid repeating the names of every "real" skip flag.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

I ran the doc build locally.

$ SKIP_API=1 SKIP_SCALADOC=1 SKIP_PYTHONDOC=1 bundle exec jekyll build
Configuration file: .../spark/docs/_config.yml

**********************
* Skip flags enabled *
**********************
SKIP_API
SKIP_PYTHONDOC
SKIP_SCALADOC

************************
* Building error docs. *
************************
Generated: docs/_generated/error-conditions.html
            Source: .../spark/docs
       Destination: .../spark/docs/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    done in 7.553 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

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

No.