### What changes were proposed in this pull request?
This PR fixes our documentation build to copy minified jquery file instead.
The original file `jquery.js` seems missing as of Scala 2.12 upgrade. Scala 2.12 seems started to use minified `jquery.min.js` instead.
Since we dropped Scala 2.11, we won't have to take care about legacy `jquery.js` anymore.
Note that, there seem multiple weird stuff in the current ScalaDoc (e.g., some pages are weird, it starts from `scala.collection.*` or some pages are missing, or some docs are truncated, some badges look missing). It needs a separate double check and investigation.
This PR targets to make the documentation generation pass in order to unblock Spark 3.0 preview.
### Why are the changes needed?
To fix and make our official documentation build able to run.
### Does this PR introduce any user-facing change?
It will enable to build the documentation in our official way.
**Before:**
```
Making directory api/scala
cp -r ../target/scala-2.12/unidoc/. api/scala
Making directory api/java
cp -r ../target/javaunidoc/. api/java
Updating JavaDoc files for badge post-processing
Copying jquery.js from Scala API to Java API for page post-processing of badges
jekyll 3.8.6 | Error: No such file or directory rb_sysopen - ./api/scala/lib/jquery.js
```
**After:**
```
Making directory api/scala
cp -r ../target/scala-2.12/unidoc/. api/scala
Making directory api/java
cp -r ../target/javaunidoc/. api/java
Updating JavaDoc files for badge post-processing
Copying jquery.min.js from Scala API to Java API for page post-processing of badges
Copying api_javadocs.js to Java API for page post-processing of badges
Appending content of api-javadocs.css to JavaDoc stylesheet.css for badge styles
...
```
### How was this patch tested?
Manually tested via:
```
SKIP_PYTHONDOC=1 SKIP_RDOC=1 SKIP_SQLDOC=1 jekyll build
```
Closes#26228 from HyukjinKwon/SPARK-29569.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: Xingbo Jiang <xingbo.jiang@databricks.com>
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