[SPARK-32774][BUILD] Don't track docs/.jekyll-cache

### What changes were proposed in this pull request?

This PR changes .gitignore not to track docs/.jekyll-cache.

### Why are the changes needed?

When I build docs, docs/.jekyll-cache can be created and it should not be tracked.
```
$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	docs/.jekyll-cache/
```

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Applied the change and confirmed the result of `git status`
```
$ git status
On branch untrack-jekyll-cache
nothing to commit, working tree clean
```

Closes apache#29622 from sarutak/untrack-jekyll-cache.

Authored-by: Kousuke Saruta <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

.gitignore

@@ -80,6 +80,7 @@ target/
 unit-tests.log
 work/
 docs/.jekyll-metadata
+docs/.jekyll-cache
 
 # For Hive
 TempStatsStore/