[SPARK-46825][DOCS] Build Spark only once when building docs

[nchammas]

What changes were proposed in this pull request?

As suggested here, this change improves the documentation build so that it builds Spark at most one time, regardless of what API docs are requested in the build.

Why are the changes needed?

There is no need to build Spark multiple times when generating docs. In particular, building Scala and Python docs, or Scala and SQL docs, causes Spark to be built twice.

Fixing this problem saves us a couple of minutes.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

I built the docs as follows on master as well as on this branch:

time SKIP_RDOC=1 SKIP_PYTHONDOC=1 bundle exec jekyll build

The time results before and after this change are as follows:

before
------
real    6m48.815s
user    23m17.943s
sys     1m29.578s

after
-----
real    4m10.672s
user    14m10.130s
sys     1m0.773s

That's a savings of about 2.5 minutes.

Additionally, I diffed the generated _site/ dir across master and this branch and confirmed they are essentially identical except for some general SQL examples files.

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

No.

[HyukjinKwon]

Merged to master.