[SPARK-32584][PYTHON][DOCS] Exclude _images and _sources that are generated by Sphinx in Jekyll build

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to include _images and _sources directories, generated from Sphinx, in Jekyll build.

For _images directory,
After SPARK-31851, now we add some images to use within the pages built by Sphinx. It copies and images into _images directory. Later, when Jekyll builds, the underscore directories are ignored by default which ends up with missing image in the main doc.

Before:

After:

For _sources directory,
Please refer here and here. They are generated by default and used by default in the documentations by Sphinx, and we should better include them.

Why are the changes needed?

To show the images correctly in PySpark documentation.

Does this PR introduce any user-facing change?

No, only in unreleased branches.

How was this patch tested?

Manually tested via:

SKIP_SCALADOC=1 SKIP_RDOC=1 SKIP_SQLDOC=1 jekyll serve --watch

[HyukjinKwon]

cc @viirya can you take a quick look when you're available?

[SparkQA]

Test build #127307 has finished for PR 29402 at commit 3cb7891.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

lgtm

[HyukjinKwon]

Thanks @viirya! Merged to master.