[SPARK-57938][INFRA] Add new SKIP_ flags to doc build script

[nchammas]

What changes were proposed in this pull request?

Add a couple of new SKIP_ flags to the existing doc build script:

• SKIP_SPARK_BUILD
• SKIP_UNIDOC

Update the docs README to reflect the complete set of skip flags.

Change the docs build script to accept any non-empty value as enabling a particular skip flag.

Why are the changes needed?

#48860 added SPARK_DOCS_IS_BUILT_ON_HOST but it doesn't really fit the existing "interface" of the doc build script, as clunky as it is. (The interface is basically a collection of SKIP_ flags.)

This PR tweaks the new skip flags so they are more consistent with the existing ones.

The change from accepting 1 only to accepting any non-empty value also makes the docs build more consistent with how skip flags are handled elsewhere, like here.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

I ran the doc build as follows without issue:

SKIP_UNIDOC=1 SKIP_SPARK_BUILD=1 SKIP_RDOC=1 bundle exec jekyll build

The doc build worked as expected. The Spark build was correctly skipped, and the Scala/Java doc build was quick since it was just copying files around.

I also ran the docs with other values passed in to the skip flags and confirmed they are accepted:

$ SKIP_SPARK_BUILD=true SKIP_RDOC=fart SKIP_SCALADOC=wow SKIP_PYTHONDOC=no SKIP_SQLDOC=yes bundle exec jekyll build
Configuration file: /Users/nicholaschammas/Projects/nchammas/spark/docs/_config.yml

**********************
* Skip flags enabled *
**********************
SKIP_PYTHONDOC
SKIP_RDOC
SKIP_SCALADOC
SKIP_SPARK_BUILD
SKIP_SQLDOC

***********************
* Building error docs *
***********************
Using Python 3.11.9.

<snipped>

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

No.

[nchammas]

cc @panbingkun, who authored the referenced PR.

[uros-b]

Please check possible latent inconsistency: SKIP_UNIDOC reuses a name that already exists cross-subsystem; dev/run-tests.py:307 reads it via truthiness and multiple non-doc CI jobs set SKIP_UNIDOC: true, whereas the doc script gates strictly on == '1'. No live conflict (the docs job at lines 1246-1577 sets neither new flag, so it builds both as before, and no CI path both exports SKIP_UNIDOC=true and runs the Jekyll build), but a user exporting SKIP_UNIDOC=true expecting the doc build to skip would be surprised.

[nchammas]

I've updated the doc build to accept any non-empty value for its skip flags. I've updated the PR description accordingly.

[uros-b]

Doc-wording nit: the SKIP_UNIDOC=1 entry ("Skip building the Unidoc") omits that the Scala/Java copy step (copy_and_update_scala_docs/copy_and_update_java_docs from target/scala-2.13/unidoc and target/javaunidoc) still runs, so the build fails there if no pre-built unidoc output exists. SKIP_SPARK_BUILD=1 carries the analogous "only set this if you've already built" caveat; SKIP_UNIDOC should too (use SKIP_SCALADOC=1 to skip the copy as well).

[uros-b]

Thank you @nchammas, this is a clean, well-scoped INFRA refactor that makes the doc-build skip interface consistent and more granular. I left a few nit comments, but otherwise LGTM.